Method for manufacturing semiconductor device

ABSTRACT

Electrical characteristics of transistors using an oxide semiconductor are greatly varied in a substrate, between substrates, and between lots, and the electrical characteristics are changed due to heat, bias, light, or the like in some cases. In view of the above, a semiconductor device using an oxide semiconductor with high reliability and small variation in electrical characteristics is manufactured. In a method for manufacturing a semiconductor device, hydrogen in a film and at an interface between films is removed in a transistor using an oxide semiconductor. In order to remove hydrogen at the interface between the films, the substrate is transferred under a vacuum between film formations. Further, as for a substrate having a surface exposed to the air, hydrogen on the surface of the substrate may be removed by heat treatment or plasma treatment.

TECHNICAL FIELD

The present invention relates to a film formation apparatus and a methodfor manufacturing a semiconductor device using the film formationapparatus.

Note that in this specification, a semiconductor device refers to anydevice that can function by utilizing semiconductor characteristics, andan electro-optical device, a semiconductor circuit, and an electronicdevice are all semiconductor devices.

BACKGROUND ART

In recent years, attention has been focused on a technique for formingtransistors using semiconductor thin films formed over a substratehaving an insulating surface. The transistors are applied to a widerange of electronic devices such as an integrated circuit (IC) or animage display device (display device). As materials of semiconductorthin films applicable to the transistors, silicon-based semiconductormaterials have been widely known, but oxide semiconductors have beenattracting attention as alternative materials.

For example, disclosure is made of a transistor having an active layerfor which an oxide semiconductor that contains indium (In), gallium(Ga), and zinc (Zn) and has an electron carrier concentration less than10¹⁸/cm³ is used, and a sputtering method is considered the mostsuitable as a method for forming a film of the oxide semiconductor (seePatent Document 1).

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2006-165528

DISCLOSURE OF INVENTION

Electrical characteristics of transistors using an oxide semiconductorare greatly varied in a substrate, between substrates, and between lots,and the electrical characteristics are changed due to heat, bias, light,or the like in some cases. In view of the above, an object is tomanufacture a semiconductor device using an oxide semiconductor withhigh reliability and small variation in electrical characteristics.

It is known that in a transistor using an oxide semiconductor, part ofhydrogen serves as a donor to generate an electron. The generation of anelectron causes drain current to flow even without applying a gatevoltage; thus, the threshold voltage shifts in the negative direction. Atransistor using an oxide semiconductor is likely to have n-typeconductivity, and it comes to have normally-on characteristics by ashift of threshold voltage in the negative direction. Here, “normallyon” means a state where a channel exists without applying voltage to agate electrode and current flows through a transistor.

Further, the threshold voltage of a transistor might change due to entryof hydrogen into an oxide semiconductor film after manufacturing thetransistor. A change in the threshold voltage significantly impairs thereliability of the transistor.

The present inventor has found that film formation causes unintendedinclusion of hydrogen in a film. Note that in this specification,“hydrogen” refers to a hydrogen atom or a hydrogen ion and for example,includes hydrogen derived from a hydrogen molecule, hydrocarbon, ahydroxyl group, water, and the like in the expression “includinghydrogen”.

One embodiment of the present invention is a method for manufacturing asemiconductor device by which hydrogen in a film and at an interfacebetween films is removed in a transistor using an oxide semiconductor.In order to remove hydrogen at an interface between films, a substrateis not exposed to the air between film formations. The substrate ispreferably transferred under a vacuum. Further, as for a substratehaving a surface exposed to the air, hydrogen on the surface of thesubstrate may be removed by heat treatment or plasma treatment.

In order to remove hydrogen in a film, hydrogen on a surface of asubstrate over which a film is to be formed, hydrogen in a material ofthe film, and hydrogen in a film formation chamber are reduced.

Hydrogen taken into the film may be removed by heat treatment or plasmatreatment.

Heat treatment or plasma treatment may be performed in order to reducethe hydrogen concentration of the surface of the substrate over which afilm is to be formed and the hydrogen concentration of the film.

In one embodiment of the present invention, heat treatment is performedin an inert atmosphere, a reduced-pressure atmosphere, or a dry airatmosphere. Further, the inert atmosphere refers to an atmospherecontaining an inert gas (such as nitrogen or a rare gas (e.g., helium,neon, argon, krypton, or xenon)) as the main component, and preferablycontains no hydrogen. For example, the purity of the inert gas to beintroduced is greater than or equal to 8N (99.999999%), preferablygreater than or equal to 9N (99.9999999%). Alternatively, the inertatmosphere refers to an atmosphere that contains an inert gas as themain component and contains a reactive gas at a concentration less than0.1 ppm. The reactive gas is a gas that reacts with a semiconductor,metal, or the like. The reduced-pressure atmosphere refers to anatmosphere with a pressure of lower than or equal to 10 Pa. The dry airatmosphere is a dew point lower than or equal to −40° C., preferably adew point lower than or equal to −50° C.

The plasma treatment can be performed at low temperature and can removehydrogen efficiently in a short time. In particular, the plasmatreatment is effective in removing hydrogen strongly bonded to a surfaceof a substrate.

Further, entry of hydrogen from the outside can be suppressed by filmsbetween which a transistor is interposed and which block hydrogen.

According to one embodiment of the present invention, hydrogen containedin an oxide semiconductor film can be reduced, and a transistor havingstable electrical characteristics with less variation in thresholdvoltage can be provided.

Alternatively, according to one embodiment of the present invention,hydrogen in a film in contact with an oxide semiconductor film can bereduced, and thus entry of hydrogen into the oxide semiconductor filmcan be suppressed. Consequently, a semiconductor device having atransistor with favorable electrical characteristics and highreliability can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart showing a path of a substrate in a process ofmanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 2A to 2D are cross-sectional views showing an example of a processof manufacturing a semiconductor device that is one embodiment of thepresent invention.

FIG. 3 is a top view showing an example of a film formation apparatusthat is one embodiment of the present invention.

FIGS. 4A to 4D are cross-sectional views showing an example of amanufacturing process of a semiconductor device that is one embodimentof the present invention.

FIGS. 5A to 5C are a top view and cross-sectional views showing anexample of a semiconductor device that is one embodiment of the presentinvention.

FIG. 6 is a flow chart showing a path of a substrate in a process ofmanufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 7A to 7C are cross-sectional views showing an example of a processof manufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 8A to 8E are cross-sectional views showing an example of a processof manufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 9A to 9C are cross-sectional views showing an example of a processof manufacturing a semiconductor device that is one embodiment of thepresent invention.

FIGS. 10A to 10E are cross-sectional views showing an example of aprocess of manufacturing a semiconductor device that is one embodimentof the present invention.

FIG. 11 is a circuit diagram showing an example of a pixel in a displaydevice that is one embodiment of the present invention.

FIGS. 12A and 12B are a top view and a cross-sectional view showing anexample of a pixel in a display device that is one embodiment of thepresent invention.

FIGS. 13A to 13F are cross-sectional views showing an example of aprocess of manufacturing a pixel in a display device that is oneembodiment of the present invention.

FIGS. 14A to 14C are cross-sectional views showing an example of aprocess of manufacturing an oxide semiconductor film that is oneembodiment of the present invention.

FIGS. 15A and 15B are a top view and a cross-sectional view showing anexample of a display device that is one embodiment of the presentinvention.

FIGS. 16A to 16F each show an example of an electronic device that isone embodiment of the present invention.

FIG. 17 is a cross-sectional view showing an example of a semiconductordevice that is one embodiment of the present invention.

FIG. 18 is a top view showing an example of a film formation apparatusthat is one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Further, the present inventionis not construed as being limited to description of the followingembodiments. In describing structures of the present invention withreference to the drawings, the same reference numerals are used incommon for the same portions in different drawings. Note that the samehatch pattern is applied to similar parts, and the similar parts are notespecially denoted by reference numerals in some cases.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

(Embodiment 1)

In this embodiment, description is made on a method for manufacturing atop-gate top-contact transistor using an oxide semiconductor with lessentry of hydrogen into a film and an interface between films.

FIG. 1 is a flow chart showing a path of a substrate in a multi-chamberfilm formation apparatus. FIGS. 2A to 2D are cross-sectional views thatshows a method for manufacturing a semiconductor device and correspondsto the flow diagram in FIG. 1.

FIG. 3 is a multi-chamber film formation apparatus. The film formationapparatus includes a substrate supply chamber 11 having three cassetteports 14 accommodating a substrate, a load lock chamber 12 a, a loadlock chamber 12 b, a transfer chamber 13, a substrate processing chamber15, a film formation chamber 10 a with a leakage rate less than or equalto 1×10⁻¹⁰ Pa·m³/sec, a film formation chamber 10 b with a leakage rateless than or equal to 1×10⁻¹⁰ Pa·m³/sec, and a film formation chamber 10c with a leakage rate less than or equal to 1×10⁻¹⁰ Pa·m³/sec. Thesubstrate supply chamber is connected to the load lock chamber 12 a andthe load lock chamber 12 b. The load lock chamber 12 a and the load lockchamber 12 b are connected to the transfer chamber 13. The substrateprocessing chamber 15 and the film formation chambers 10 a to 10 c areeach connected only to the transfer chamber 13. A gate valve is providedfor a connecting portion of each chamber so that each chamber can beindependently kept in a vacuum state. Note that a film formation gashaving a purity greater than or equal to 99.999999% can be introducedinto the film formation chambers 10 a to 10 c. Although not shown, thetransfer chamber 13 has one or more substrate transfer robots. Here, theatmosphere in the substrate processing chamber 15 can be controlled tobe the one containing almost no hydrogen (e.g., an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere); for example, adry nitrogen atmosphere having a dew point of lower than or equal to−40° C., preferably lower than or equal to −50° C., is possible. Here,the substrate processing chamber 15 preferably also serves as asubstrate heating chamber and a plasma treatment chamber. With a singlewafer multi-chamber film formation apparatus, a substrate does not needto be exposed to air between treatments, and adsorption of hydrogen to asubstrate can be suppressed. In addition, the order of film formation,heat treatment, or the like can be freely created. Note that the numbersof the film formation chambers, the load lock chambers, and thesubstrate processing chambers are not limited to the above numbers, andcan be determined as appropriate depending on the space for placement orthe process.

First, as shown in a step S501 in FIG. 1, a substrate 100 is put on thecassette port 14 in the substrate supply chamber 11.

Next, as shown in a step S502 in FIG. 1, a gate valve of the load lockchamber 12 a set to an atmospheric pressure state is opened, thesubstrate 100 is transferred from the cassette port 14 to the load lockchamber 12 a with a first transfer robot, and then the gate valve isclosed.

The load lock chamber 12 a is evacuated and set to a vacuum state afterthe substrate 100 is introduced into the load lock chamber 12 a. Asshown in a step S503 in FIG. 1, a gate valve between the load lockchamber 12 a in the vacuum state and the transfer chamber 13 in a vacuumstate is opened, the substrate 100 is transferred to the transferchamber 13 with a second transfer robot, and then the gate valve isclosed.

After the substrate 100 is introduced into the transfer chamber 13, asshown in a step S504 in FIG. 1, a gate valve between the transferchamber 13 and the substrate processing chamber 15 in a vacuum state isopened, the substrate 100 is transferred to the substrate processingchamber 15 with the second robot, and then the gate valve is closed.

After the substrate 100 is introduced into the substrate processingchamber 15, the substrate 100 is subjected to heat treatment or plasmatreatment (see FIG. 2A). By subjecting the substrate 100 to the heattreatment or the plasma treatment, the substrate 100 can be dehydratedor dehydrogenated. The heat treatment or the plasma treatment isperformed at a temperature higher than or equal to 300° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 400° C. and lower than or equal to 550° C. in an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. A resistanceheating method or the like may be used for the heating. Alternatively,rapid thermal anneal (RTA) treatment, such as gas rapid thermal anneal(GRTA) treatment or lamp rapid thermal anneal (LRTA) treatment, can beused. The LRTA treatment is treatment for heating an object by radiationof light (an electromagnetic wave) emitted from a lamp, such as ahalogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp,a high-pressure sodium lamp, or a high-pressure mercury lamp. A GRTAapparatus is an apparatus for performing heat treatment using ahigh-temperature gas. As the gas, an inert gas is used. When heattreatment is performed in a short time using RTA, it is possible not towarp the substrate even at a temperature higher than or equal to thestrain point of the substrate, so that the substrate can be dehydratedand dehydrogenated efficiently. For example, the substrate temperaturemay be higher than or equal to 500° C. and lower than or equal to 650°C. and the treatment time may be higher than or equal to 1 minute andless than or equal to 10 minutes. By plasma treatment, hydrogen absorbedon a surface of the substrate can be removed with plasma generated in anatmosphere of a rare gas, oxygen, nitrogen, or the like. Further, byplasma treatment, hydrogen which is strongly bonded to the substrate 100can be removed efficiently. For example, argon plasma treatment may beperformed by a reverse sputtering method.

After the dehydration or the dehydrogenation of the substrate 100, asshown in a step S505 in FIG. 1, the gate valve between the transferchamber 13 and the substrate processing chamber 15 is opened, thesubstrate 100 is transferred to the transfer chamber 13 with the secondtransfer robot, and then the gate valve is closed.

After the substrate 100 is introduced into the transfer chamber 13, agate valve between the transfer chamber 13 and the film formationchamber 10 c in a vacuum state is opened, as shown in a step S506 inFIG. 1, the substrate 100 is transferred to the film formation chamber10 c with the second transfer robot, and then the gate valve is closed.

After the substrate 100 is introduced into the film formation chamber 10c, a base insulating film 102 is formed to have a thickness of greaterthan or equal to 100 nm and less than or equal to 500 nm over thesubstrate 100 (see FIG. 2B). The base insulating film 102 is formed by afilm formation method such as a sputtering method, a molecular beamepitaxy (MBE) method, a CVD method, a pulse laser deposition method, oran atomic layer deposition (ALD) method.

A single layer or a stacked layer of silicon oxide, silicon oxynitride,silicon nitride, silicon nitride oxide, gallium oxide, gallium aluminumoxide (Ga_(x)Al_(2−x)O_(3+y) (x is greater than or equal to 0 and lessthan or equal to 2, and y is greater than 0 and less than 1)), aluminumoxide, aluminum oxynitride, aluminum nitride oxide, aluminum nitride, orthe like is used for a material of the base insulating film 102. Forexample, the base insulating film 102 has a layered structure of asilicon nitride film and a silicon oxide film, so that entry of moistureinto a transistor 150 from the substrate or the like can be prevented.When the base insulating film 102 is formed to have a layered structure,a film which is in contact with an oxide semiconductor film 106 formedlater may be an insulating film (e.g., silicon oxide, siliconoxynitride, or aluminum oxide) from which oxygen is released by heating.Thus, oxygen is supplied from the base insulating film 102 to the oxidesemiconductor film 106, so that oxygen deficiency in the semiconductorfilm 106 and the interface state between the base insulating film 102and the oxide semiconductor film 106 can be reduced. The oxygendeficiency of the oxide semiconductor film 106 causes the thresholdvoltage to shift in the negative direction, and the interface statebetween the base insulating film 102 and the oxide semiconductor film106 reduces the reliability of the transistor.

Note that here, silicon oxynitride refers to silicon that includes moreoxygen than nitrogen. In the case where measurements are performed usingRutherford backscattering spectrometry (RBS) and hydrogen forwardscattering spectrometry (HFS), silicon oxynitride includes oxygen,nitrogen, and silicon at concentrations ranging from 50 at. % to 70 at.%, 0.5 at. % to 15 at. %, and 25 at. % to 35 at. % respectively.Further, silicon nitride oxide means silicon that includes more nitrogenthan oxygen. In the case where measurements are conducted using RBS andHFS, silicon nitride oxide preferably contains oxygen, nitrogen,silicon, and hydrogen at concentrations ranging from 5 at. % to 30 at.%, 20 at. % to 55 at. %, 25 at. % to 35 at. %, and 10 at. % to 30 at. %respectively. Note that percentages of nitrogen, oxygen, silicon, andhydrogen fall within the ranges given above, where the total number ofatoms contained in the silicon oxynitride film or the silicon nitrideoxide film is defined as 100 at. %.

Aluminum oxynitride refers to a substance that contains more oxygen thannitrogen. Further, aluminum nitride oxide refers to a substance thatcontains more nitrogen than oxygen.

The “insulating film that releases oxygen by heating” refers to aninsulating film from which the amount of released oxygen is greater thanor equal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to1.0×10²⁰ atoms/cm³, further preferably greater than or equal to 3.0×10²⁰atoms/cm³ when converted into oxygen atoms by thermal desorptionspectroscopy (TDS) analysis.

Here, a method in which the amount of released oxygen is measured bybeing converted into oxygen atoms using TDS analysis will be described.

The amount of released gas in TDS analysis is proportional to theintegral value of a spectrum. Therefore, the amount of released gas canbe calculated from the ratio between the integral value of a spectrum ofan insulating film and the reference value of a standard sample. Thereference value of a standard sample refers to the ratio of the densityof a predetermined atom contained in a sample to the integral value of aspectrum.

For example, the number of the released oxygen molecules (N_(O2)) froman insulating film can be found according to an equation 1 with the TDSanalysis results of a silicon wafer containing hydrogen at apredetermined density which is the standard sample and the TDS analysisresults of the insulating film. Here, all spectra having a mass numberof 32 which are obtained by the TDS analysis are assumed to originatefrom an oxygen molecule. CH₃OH, which is given as a gas having a massnumber of 32, is not taken into consideration on the assumption that itis unlikely to be present. Further, an oxygen molecule including anoxygen atom having a mass number of 17 or 18 which is an isotope of anoxygen atom is also not taken into consideration because the proportionof such a molecule in the natural world is minimal.N_(O2)=N_(H2)/S_(H2)×S_(O2)×α  (Equation 1)

N_(H2) is a value obtained by conversion of the number of hydrogenmolecules desorbed from the standard sample into density. S_(H2) is anintegral value of spectrum of a standard sample which is analyzed byTDS. Here, the reference value of the standard sample is set toN_(H2)/S_(H2). S_(O2) is the integral value of a spectrum when theinsulating film is subjected to TDS analysis. α is a coefficient whichinfluences spectrum intensity in TDS analysis. Refer to JapanesePublished Patent Application No. H6-275697 for details of theEquation 1. Note that the amount of released oxygen from the aboveinsulating film is measured with a thermal desorption spectroscopyapparatus produced by ESCO Ltd., EMD-WA1000S/W using a silicon wafercontaining a hydrogen atom at 1×10¹⁶ atoms/cm³ as the standard sample.

Further, in the TDS analysis, oxygen is partly detected as an oxygenatom. The ratio between oxygen molecules and oxygen atoms can becalculated from the ionization rate of the oxygen molecules. Note that,since the above a includes the ionization rate of the oxygen molecules,the number of the released oxygen atoms can also be estimated throughthe evaluation of the number of the released oxygen molecules.

Note that N_(O2) is the number of the released oxygen molecules. For theinsulating film, the amount of released oxygen when converted intooxygen atoms is twice the number of the released oxygen molecules.

In the above structure, the insulating film that releases oxygen byheating may be oxygen-excess silicon oxide (SiO_(X) (X>2)). In theoxygen-excess silicon oxide (SiO_(X) (X>2)), the number of oxygen atomsper unit volume is more than twice the number of silicon atoms per unitvolume. The number of silicon atoms and the number of oxygen atoms perunit volume are measured by Rutherford backscattering spectrometry.

In the case where a mixed gas of oxygen and a rare gas is used as a filmformation gas when the insulating layer from which oxygen is released byheating is formed by a sputtering method, the ratio of oxygen to therare gas is preferably high. For example, the oxygen concentration inthe whole gas may be greater than or equal to 6% and less than or equalto 100%. An oxide target is preferably used.

After the base insulating film 102 is formed over the substrate 100, asshown in a step S507 in FIG. 1, the gate valve between the transferchamber 13 and the film formation chamber 10 c is opened, the substrate100 is transferred to the transfer chamber 13 with the second transferrobot, and then the gate valve is closed.

Although not shown, then, the gate valve between the transfer chamber 13and the substrate processing chamber 15 in the vacuum state is opened,the substrate 100 is transferred to the substrate processing chamber 15with the second transfer robot, the gate valve is closed, and then thesubstrate 100 may be subjected to heat treatment. The heat treatment isperformed at a temperature of higher than or equal to 150° C. and lowerthan or equal to 280° C., preferably higher than or equal to 200° C. andlower than or equal to 250° C. in an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. Through the above,hydrogen can be removed from the substrate 100 and the base insulatingfilm 102. Note that the temperature at which hydrogen is removed fromthe base insulating film 102 but oxygen is released as little aspossible is preferable. After the substrate 100 is subjected to the heattreatment, the gate valve between the transfer chamber 13 and thesubstrate processing chamber 15 is opened, the substrate 100 istransferred to the transfer chamber 13 with the second transfer robot,and then the gate valve is closed.

After the substrate 100 is introduced into the transfer chamber 13, agate valve between the transfer chamber 13 and the film formationchamber 10 a in a vacuum state is opened, as shown in a step S508 inFIG. 1, the substrate 100 is transferred to the film formation chamber10 a with the second transfer robot, and then the gate valve is closed.

After the substrate 100 is introduced into the film formation chamber 10a, the oxide semiconductor film 106 is formed to have a thickness ofgreater than or equal to 3 nm and less than or equal to 50 nm over thebase insulating film 102 (see FIG. 2C). The oxide semiconductor film 106is formed by a film formation method such as a sputtering method, a MBEmethod, a CVD method, a pulse laser deposition method, or an ALD method.

As a material used for the oxide semiconductor film 106, afour-component metal oxide such as an In—Sn—Ga—Zn—O-based material; athree-component metal oxide such as an In—Ga—Zn—O-based material, anIn—Sn—Zn—O-based material, an In—Al—Zn—O-based material, aSn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or aSn—Al—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; an In—O-basedmaterial; a Sn—O-based material; a Zn—O-based material; or the like maybe used. In addition, any of the above materials may contain siliconoxide. Here, for example, an In—Ga—Zn—O-based material means an oxideincluding indium (In), gallium (Ga), and zinc (Zn), and there is noparticular limitation on the composition ratio. Further, theIn—Ga—Zn—O-based material may contain an element other than In, Ga, andZn.

As the oxide semiconductor film 106, a thin film using a materialrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0), may beformed. Here, M represents one or more metal elements selected from Ga,Al, Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Ga andCo, or the like.

An alkali metal and an alkaline earth metal are adverse impurities foran oxide semiconductor and are preferably contained little. Theconcentration of Na is less than or equal to 5×10¹⁶ cm⁻³, preferablyless than or equal to 1×10¹⁶ cm⁻³, more preferably less than or equal to1×10¹⁵ cm³. The concentration of Li is less than or equal to 5×10¹⁵ cm³,preferably less than or equal to 1×10¹⁵ cm³. The concentration of K isless than or equal to 5×10¹⁵ cm³, preferably less than or equal to1×10¹⁵ cm³. An alkali metal, in particular, sodium diffuses into aninsulating film and becomes Na⁺ when an insulating film in contact withthe oxide semiconductor is an oxide. In addition, sodium cuts the bondbetween a metal and oxygen or enters the bond in the oxidesemiconductor. As a result, deterioration of transistor characteristics(e.g., the shift of a threshold voltage to the negative side (causingthe transistor to be normally on) or a decrease in field-effectmobility) is caused. In addition, this also causes variation in thecharacteristics. Such a problem is significant especially in the casewhere the hydrogen concentration in the oxide semiconductor is extremelylow. Therefore, the concentration of an alkali metal is stronglyrequired to set to the above value in the case where the hydrogenconcentration in the oxide semiconductor is less than or equal to 5×10¹⁹cm⁻³, particularly less than or equal to 5×10¹⁸ cm⁻³.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide target.

As the In—Ga—Zn—O-based oxide target, for example, an oxide targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can beused. Note that it is not necessary to limit the material and thecomposition ratio of the target to the above. For example, an oxidetarget having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]can be used.

The relative density of the oxide target is greater than or equal to 90%and less than or equal to 100%, preferably greater than or equal to 95%and less than or equal to 99.9%. This is because, with the use of theoxide target with a high relative density, the formed oxidesemiconductor film can be a dense film.

For example, the oxide semiconductor film can be formed as follows.However, the present invention is not limited to the following method.

An example of the film formation conditions is as follows: the distancebetween the substrate and the target is 60 mm; the pressure is 0.4 Pa;the direct-current (DC) power is 0.5 kW; and the film formationatmosphere is a mixed atmosphere containing argon and oxygen (the flowrate of the oxygen is 33%). Note that a pulse direct current (DC)sputtering method is preferably used because powder substances (alsoreferred to as particles or dust) generated in film formation can bereduced and the film thickness can be uniform.

After the oxide semiconductor film 106 is formed over the substrate 100,as shown in a step S509 in FIG. 1, a gate valve between the transferchamber 13 and the film formation chamber 10 a is opened, the substrate100 is transferred to the transfer chamber 13 with the second transferrobot, and then the gate valve is closed.

After the substrate 100 is introduced into the transfer chamber 13, thegate valve between the transfer chamber 13 and the substrate processingchamber 15 in a vacuum state is opened, as shown in a step S510 in FIG.1, the substrate 100 is transferred to the substrate processing chamber15 with the second transfer robot, and then the gate valve is closed.

After the substrate 100 is introduced into the substrate processingchamber 15, the substrate 100 is subjected to heat treatment or plasmatreatment. The oxide semiconductor film 106 can be dehydrated ordehydrogenated by the heat treatment or the plasma treatment. The heattreatment or the plasma treatment is performed at temperature higherthan or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 470° C. in an oxidizing atmosphere, an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. At this time,oxygen may be supplied to the oxide semiconductor film 106 from the baseinsulating film 102 while hydrogen is removed from the oxidesemiconductor film 106. Note that the heat treatment is preferablyperformed at a temperature higher than the temperature of the heattreatment performed between the steps S507 and S508 in FIG. 1 by 5° C.or higher. By performing the heat treatment in such a temperature range,oxygen can be supplied from the base insulating film 102 to the oxidesemiconductor film 106 efficiently.

Note that an oxidizing atmosphere refers to an atmosphere containing anoxidation gas. Oxidation gas is oxygen, ozone, nitrous oxide, or thelike, and it is preferable that the oxidation gas does not containwater, hydrogen, and the like. For example, the purity of oxygen, ozone,or nitrous oxide to be introduced to a heat treatment apparatus isgreater than or equal to 8N (99.999999%), preferably greater than orequal to 9N (99.9999999%). As the oxidation gas atmosphere, anatmosphere in which an oxidation gas is mixed with an inert gas may beused, and the oxidation gas is contained at least at 10 ppm in theatmosphere.

After the oxide semiconductor film 106 is subjected to dehydrationtreatment or dehydrogenation treatment, as shown in a step S511 in FIG.1, the gate valve between the transfer chamber 13 and the substrateprocessing chamber 15 is opened, the substrate 100 is transferred to thetransfer chamber 13 with the second transfer robot, and then the gatevalve is closed.

After the substrate 100 is introduced into the transfer chamber 13, asshown in a step S512 in FIG. 1, a gate valve between the transferchamber 13 and the film formation chamber 10 b in a vacuum state isopened, the substrate 100 is transferred to the film formation chamber10 b with the second transfer robot, and then the gate valve is closed.

After the substrate 100 is introduced into the film formation chamber 10b, an oxide conductive film 128 is formed over the oxide semiconductorfilm 106 to have a thickness of greater than or equal to 3 nm and lessthan or equal to 30 nm (see FIG. 2D). The oxide conductive film 128 isformed by a film formation method such as a sputtering method, a MBEmethod, a CVD method, a pulse laser deposition method, or an ALD method.

By the provision of the oxide conductive film between the oxidesemiconductor film 106 and a source electrode 108 a formed later andbetween the oxide semiconductor film 106 and a drain electrode 108 bformed later, it is possible to reduce the contact resistance between asource region and the oxide semiconductor film 106 and between a drainregion and the oxide semiconductor film 106, so that the transistor canoperate at high speed.

As the oxide conductive film, indium oxide (In₂O₃), tin oxide (SnO₂),zinc oxide (ZnO), indium tin oxide (In₂O₃—SnO₂; abbreviated to ITO),indium zinc oxide (In₂O₃—ZnO), or any of these metal oxide materials inwhich silicon oxide is contained can be used.

Alternatively, the oxide conductive film may be formed by injection ofnitrogen into the oxide semiconductor film 106. Alternatively, the oxideconductive film may be formed with the use of a material similar to thatof the oxide semiconductor film 106 as a target by a sputtering methodin which nitrogen is included in a film formation gas.

Note that all of the steps are not necessarily undergone in the path ofthe substrate shown in FIG. 1. For example, one or more of the stepsS504, S510, and S512 in FIG. 1 may be omitted. In that case, followingsteps may be changed as appropriate.

The above steps are performed without exposure to the air.

Although not shown in the flow of FIG. 1, after the oxide conductivefilm 128 is formed, the substrate 100 is returned to the cassette port14 in the substrate supply chamber 11 through the transfer chamber 13and the load lock chamber 12 b.

Next, the following process of manufacturing the transistor is describedwith reference to FIGS. 4A to 4D.

In FIG. 2D, a resist mask is formed over the oxide conductive film 128through a photolithography step, the oxide semiconductor film 106 andthe oxide conductive film 128 are each processed to have an islandshape, and then the resist mask is removed (see FIG. 4A).

Next, a conductive film covering the island-shaped oxide semiconductorfilm 106 and the island-shaped oxide conductive film 128 is formed tohave a thickness of greater than or equal to 15 nm and less than orequal to 700 nm. A resist mask is formed through a photolithography stepand the conductive film is processed to form the source electrode 108 aand the drain electrode 108 b. At the same time, the oxide conductivefilm 128 is processed, and the resist mask is removed (see FIG. 4B).Although not shown, when the oxide conductive film 128 is processed,part of the oxide semiconductor film 106 is etched in a region betweenthe source electrode 108 a and the drain electrode 108 b in some cases.In that case, processing conditions are preferably set so that the oxidesemiconductor film 106 is not removed in the region between the sourceelectrode 108 a and the drain electrode 108 b.

As the conductive film serving as the source electrode 108 a and thedrain electrode 108 b, for example, a metal film containing an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W or a metal nitride filmcontaining any of the above elements as the main component (e.g., atitanium nitride film, a molybdenum nitride film, or a tungsten nitridefilm) can be used. A structure may be used in which a film ofhigh-melting-point metal, such as Ti, Mo, or W, or a metal nitride filmof any of these elements (e.g., a titanium nitride film, a molybdenumnitride film, or a tungsten nitride film) is stacked on one or both of alower side and an upper side of a metal film of Al, Cu, or the like.Note that the conductive film serving as the source electrode 108 a andthe drain electrode 108 b may be formed by using the multi-chamber filmformation apparatus shown in FIG. 3.

The conductive film may be processed by etching with the use of a resistmask. Ultraviolet, a KrF laser light, an ArF laser light, or the like ispreferably used for light exposure for forming a resist mask for theetching.

In the case where light exposure is performed so that the channel lengthL is less than 25 nm, the light exposure at the time of forming theresist mask is preferably performed using, for example, extremeultraviolet having an extremely short wavelength of several nanometersto several tens of nanometers. In the light exposure by extremeultraviolet light, the resolution is high and the focus depth is large.Thus, the channel length L of the transistor formed later can bereduced, whereby the operation speed of a circuit can be increased.

Etching may be performed with the use of a resist mask formed using aso-called multi-tone mask. A resist mask formed using a multi-tone maskhas a plurality of thicknesses and can be further changed in shape byashing; thus, such a resist mask can be used in a plurality of etchingsteps for different patterns. For this reason, a resist maskcorresponding to at least two kinds of different patterns can be formedby using one multi-tone mask. That is, the steps can be simplified.

Next, a gate insulating film 112 part of which is in contact with thebase insulating film 102, the oxide semiconductor film 106, and theoxide conductive film 128 and which covers the source electrode 108 aand the drain electrode 108 b is formed (see FIG. 4C).

Note that plasma treatment using an oxidation gas may be performed justbefore the formation of the gate insulating film 112 so that an exposedsurface of the oxide semiconductor film 106 is oxidized and oxygendeficiency is filled. When performed, the plasma treatment preferablyfollows the formation of the gate insulating film 112 which is to be incontact with part of the oxide semiconductor film 106 without exposureto the air. The gate insulating film 112 may be formed by using themulti-chamber film formation apparatus shown in FIG. 3.

The gate insulating film 112 can have a structure similar to that of thebase insulating film 102, and is preferably an insulating film fromwhich oxygen is released by heating. At this time, a material having ahigh dielectric constant, such as yttrium oxide, zirconium oxide,hafnium oxide, or aluminum oxide may be used for the gate insulatingfilm 112 in consideration of the function of the gate insulating film ofthe transistor. Alternatively, a stacked layer of silicon oxide, siliconoxynitride, or silicon nitride and a material having a high dielectricconstant, such as yttrium oxide, zirconium oxide, hafnium oxide, oraluminum oxide, may be used in consideration of the gate withstandvoltage and the condition of the interface between the oxidesemiconductor film and the gate insulating film 112, or the like. Thetotal thickness of the gate insulating film 112 is preferably greaterthan or equal to 1 nm and less than or equal to 300 nm, more preferablygreater than or equal to 5 nm and less than or equal to 50 nm. As thethickness of the gate insulating film is larger, a short channel effectis enhanced more and the threshold voltage tends to easily shift in thenegative direction. In addition, leakage due to a tunnel current isfound to be increased with a thickness of the gate insulating film ofless than or equal to 5 nm.

Next, a conductive film is formed. The conductive film is processed withthe use of a resist mask formed through a photolithography step, so thata gate electrode 114 is formed, and then the resist mask is removed (seeFIG. 4D). The gate electrode 114 can be formed using a metal materialsuch as molybdenum, titanium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, nitride of any of these metal materials, or analloy material which contains any of these metal materials as its maincomponent. Note that the gate electrode 114 may have a single-layerstructure or a layered structure. When a material containing Al is usedfor the conductive film, the highest process temperature in thefollowing steps is preferably lower than or equal to 380° C., morepreferably lower than or equal to 350° C. When Cu is used for theconductive film, a metal material with a higher melting point than themelting point of Cu, such as Mo, Ti, or W, is preferably stacked inorder to prevent defects caused by migration or diffusion of Cuelements. Further, when a material containing Cu is used for theconductive film, the highest process temperature in the following stepsis preferably lower than or equal to 450° C. The conductive film servingas the gate electrode 114 may be formed by using the multi-chamber filmformation apparatus shown in FIG. 3.

Note that the steps from the plasma treatment performed on the oxidesemiconductor film 106 to the formation of the conductive film servingas the gate electrode 114 are preferably performed without exposure tothe air. More preferably, the multi-chamber film formation apparatusshown in FIG. 3 is used. Without exposure to the air, hydrogen in thefilm and the interface between the films can be removed.

Through the above steps, the transistor 150 is manufactured.

FIGS. 5A to 5C show a top view and cross-sectional views of thetransistor 150.

FIG. 5B is a cross sectional view along A-B of FIG. 5A. FIG. 5C is across-sectional view along C-D of FIG. 5A. Note that in FIG. 5A, some ofthe components of the transistor 150 (e.g., the gate insulating film112) are omitted for brevity.

According to this embodiment, hydrogen in the film and the interfacebetween the films can be removed, so that a transistor with lessvariation in the threshold voltage and stable electrical characteristicsis provided. Further, hydrogen in a film in contact with an oxidesemiconductor film can be reduced, whereby entry of hydrogen into theoxide semiconductor film can be suppressed. Thus, a semiconductor devicehaving a transistor with favorable electrical characteristics and highreliability is provided.

Further, the number of apparatuses needed for manufacture of atransistor can be reduced with the use of a multi-chamber film formationapparatus.

(Embodiment 2)

In this embodiment, description is made on a method for manufacturing abottom-gate top-contact transistor using an oxide semiconductor withless entry of hydrogen into a film and an interface between films.

FIG. 6 is a flow chart showing a path of a substrate in a multi-chamberfilm formation apparatus. FIGS. 7A to 7C are cross-sectional viewscorresponding to a manufacturing flow in FIG. 6. A multi-chamber filmformation apparatus similar to that in Embodiment 1 is used.

First, as shown in a step S601 in FIG. 6, a substrate 200 is put on thecassette port 14 in the substrate supply chamber 11.

Next, as shown in a step S602 in FIG. 6, the gate valve of the load lockchamber 12 a set to an atmospheric pressure state is opened, thesubstrate 200 is transferred from the cassette port 14 to the load lockchamber 12 a with the first transfer robot, and then the gate valve isclosed.

The load lock chamber 12 a is evacuated and set to a vacuum state afterthe substrate 200 is introduced into the load lock chamber 12 a. Asshown in a step S603 in FIG. 6, the gate valve between the load lockchamber 12 a in the vacuum state and the transfer chamber 13 in a vacuumstate is opened, the substrate 200 is transferred to the transferchamber 13 with the second transfer robot, and then the gate valve isclosed.

After the substrate 200 is introduced into the transfer chamber 13, asshown in a step S604 in FIG. 6, the gate valve between the transferchamber 13 and the substrate processing chamber 15 in a vacuum state isopened, the substrate 200 is transferred to the substrate processingchamber 15 with the second transfer robot, and then the gate valve isclosed.

After the substrate 200 is introduced into the substrate processingchamber 15, the substrate 200 is subjected to heat treatment or plasmatreatment (see FIG. 7A). The heat treatment and the plasma treatment maybe performed in a manner similar to the manners in Embodiment 1.

After the substrate 200 is subjected to dehydration treatment ordehydrogenation treatment, as shown in a step S605 in FIG. 6, the gatevalve between the transfer chamber 13 and the substrate processingchamber 15 is opened, the substrate 200 is transferred to the transferchamber 13 with the second transfer robot, and then the gate valve isclosed.

After the substrate 200 is introduced into the transfer chamber 13, asshown in a step S606 in FIG. 6, the gate valve between the transferchamber 13 and the film formation chamber 10 c in a vacuum state isopened, the substrate 200 is transferred to the film formation chamber10 c with the second transfer robot, and then the gate valve is closed.

After the substrate 200 is introduced into the film formation chamber 10c, a base insulating film 202 is formed over the substrate 200 (see FIG.7B). The base insulating film 202 may have a structure similar to thatof the base insulating film 102.

After the base insulating film 202 is formed over the substrate 200, asshown in a step S607 in FIG. 6, the gate valve between the transferchamber 13 and the film formation chamber 10 c is opened, the substrate200 is transferred to the transfer chamber 13 with the second transferrobot, and then the gate valve is closed.

After the substrate 200 is introduced into the transfer chamber 13, asshown in a step S608 in FIG. 6, the gate valve between the transferchamber 13 and the film formation chamber 10 a in a vacuum state isopened, the substrate 200 is transferred to the film formation chamber10 a with the second transfer robot, and then the gate valve is closed.

After the substrate 200 is introduced into the film formation chamber 10a, a conductive film 213 is formed over the substrate 200 (see FIG. 7C).The conductive film 213 may have a structure similar to that of theconductive film serving as the gate electrode 114.

Note that all of the steps are not necessarily undergone in the path ofthe substrate shown in FIG. 6. For example, the step S604 in FIG. 6 maybe omitted. In that case, following steps may be changed as appropriate.

The above steps are performed without exposure to the air.

Although not shown in the flow of FIG. 6, after the conductive film isformed, the substrate 200 is returned to the cassette port 14 in thesubstrate supply chamber 11 through the transfer chamber 13 and the loadlock chamber 12 b.

Alternatively, after the base insulating film 202 is formed, thesubstrate 200 is transferred to the substrate processing chamber 15, thesubstrate 200 is subjected to plasma treatment or heat treatment, thesubstrate 200 is transferred to the film formation chamber 10 b throughthe transfer chamber 13, and then an insulating film may be formed. Theinsulating film may include a silicon nitride film, a silicon nitrideoxide film, a gallium oxide film, a gallium aluminum oxide film, analuminum oxide film, an aluminum oxynitride film, an aluminum nitrideoxide film, or an aluminum nitride film. Then, the substrate 200 istransferred to the film formation chamber 10 a through the transferchamber 13 and the conductive film 213 is formed. Note that the abovesteps are performed without exposure to the air. Thus, hydrogen in thefilm and the interface between the films can be further removed.

Next, a process of manufacturing the transistor following the steps inFIGS. 7A to 7C is described with reference to FIGS. 8A to 8E and FIGS.9A to 9C.

First, a resist mask is formed over the conductive film 213 through aphotolithography step, the conductive film 213 is processed, and a gateelectrode 214 is formed (see FIG. 8A).

Next, a gate insulating film 212 part of which is in contact with thebase insulating film 202 and which covers the gate electrode 214 isformed, and then an oxide semiconductor film 206 is formed over the gateinsulating film 212 (see FIG. 8B).

The gate insulating film 212 and the oxide semiconductor film 206 mayhave structures similar to those of the gate insulating film 112 and theoxide semiconductor film 106 respectively.

Next, the substrate 200 is subjected to heat treatment or plasmatreatment. By subjecting the oxide semiconductor film 206 to the heattreatment or the plasma treatment, the oxide semiconductor film 206 canbe dehydrated or dehydrogenated. The heat treatment or the plasmatreatment is performed at a temperature higher than or equal to 150° C.and lower than the strain point of the substrate, preferably higher thanor equal to 250° C. and lower than or equal to 470° C. in an oxidizingatmosphere, an inert atmosphere, a reduced-pressure atmosphere, or a dryair atmosphere. When an insulating film releasing oxygen is used as thegate insulating film 212, oxygen can be supplied from the gateinsulating film 212 to the oxide semiconductor film 206 by the heattreatment.

An oxide conductive film 228 is formed over the oxide semiconductor film206 (see FIG. 8C). The oxide conductive film 228 may have a structuresimilar to that of the oxide conductive film 128. Note that the oxideconductive film 228 is not necessarily provided.

Here, the steps from the formation of the gate insulating film 212 tothe formation of the oxide conductive film 228 are preferably performedwithout exposure to the air. Hydrogen in the film and the interfacebetween the films can be removed and the electrical characteristics andthe reliability of a transistor can be improved. Note that themulti-chamber film formation apparatus shown in FIG. 3 can be used forthe steps from the formation of the gate insulating film 212 to theformation of the oxide conductive film 228.

Next, a resist mask is formed over the oxide conductive film 228 througha photolithography step and the oxide conductive film 228 and the oxidesemiconductor film 206 are each processed to have an island shape (seeFIG. 8D).

Next, a conductive film covering the oxide conductive film 228, theoxide semiconductor film 206, and the gate insulating film 212 isformed, a resist mask is formed over the conductive film through aphotolithography step, and the conductive film is processed to form asource electrode 208 a and a drain electrode 208 b. At the same time,part of the oxide conductive film 228 between the source electrode 208 aand the drain electrode 208 b is also processed, and the oxideconductive film 228 is provided so as to be in contact with a surface ofthe oxide semiconductor film 206 and be connected to part of the sourceand drain electrodes (see FIG. 8E). The source electrode 208 a and thedrain electrode 208 b may have structures similar to those of the sourceelectrode 108 a and the drain electrode 108 b.

Through the above steps, a bottom-gate top-contact transistor 250 can bemanufactured.

Here, the source electrode 208 a, the drain electrode 208 b, the oxideconductive film 228, the oxide semiconductor film 206, and the gateinsulating film 212 may be subjected to plasma treatment or heattreatment. For example, when a reverse sputtering treatment isperformed, upper end portions of the source electrode 208 a and thedrain electrode 208 b have curved surfaces, whereby field effectconcentration at the time of operating the transistor can be relieved.Further, defects in the oxide semiconductor film 206 or in the vicinityof the surface of the oxide semiconductor film 206 are repaired byplasma treatment or heat treatment in an oxidizing atmosphere, so thatoxygen deficiency can be reduced. The heat treatment may be performed attemperature higher than or equal to 200° C. and lower than or equal to500° C.

Next, an interlayer insulating film 216 part of which is in contact withthe oxide conductive film 228, the oxide semiconductor film 206, and thegate insulating film 212 and which covers the source electrode 208 a andthe drain electrode 208 b is formed, and then a conductive film 222 isformed over the interlayer insulating film 216 (see FIG. 9A). Note thatthe interlayer insulating film 216 may have a structure similar to thatof the base insulating film 202. For example, part of the interlayerinsulating film 216 is preferably a silicon nitride film, a siliconnitride oxide film, a gallium oxide film, a gallium aluminum oxide film,an aluminum oxide film, an aluminum oxynitride film, an aluminum nitrideoxide film, or an aluminum nitride film because hydrogen entering fromthe outside can be reduced. For the conductive film 222, a materialsimilar to that of the gate electrode 214 or the oxide conductive film228 may be used. Note that after the interlayer insulating film 216 isformed, heat treatment may be performed at a temperature higher than orequal to 250° C. and lower than or equal to 350° C. in an oxidizingatmosphere, an inert atmosphere, a reduced-pressure atmosphere, or a dryair atmosphere. In the case where an insulating film releasing oxygen isused as the interlayer insulating film 216, oxygen can be supplied fromthe interlayer insulating film 216 to the oxide semiconductor film 206by performing the heat treatment. Further, oxygen deficiency at theinterface between the oxide semiconductor film 206 and the interlayerinsulating film 216 or in the vicinity of the oxide semiconductor film206 and the interlayer insulating film 216 is reduced by suppliedoxygen. In addition, when the part of the interlayer insulating film 216is a silicon nitride film, a silicon nitride oxide film, a gallium oxidefilm, a gallium aluminum oxide film, an aluminum oxide film, an aluminumoxynitride film, an aluminum nitride oxide film, or an aluminum nitridefilm, outward diffusion of the supplied oxygen can be prevented. Inother words, new oxygen deficiency is less likely to be generated in theoxide semiconductor film 206 and in the vicinity thereof; thus, thetransistor with favorable electrical characteristics and highreliability can be manufactured.

Here, the steps from the plasma treatment or the heat treatment on thesource electrode 208 a, the drain electrode 208 b, the oxide conductivefilm 228, the oxide semiconductor film 206, and the gate insulating film212 to the formation of the conductive film 222 may be performed withoutexposure to the air. Hydrogen in the film and the interface between thefilms can be removed and the electrical characteristics and thereliability of the transistor can be improved. Note that themulti-chamber film formation apparatus shown in FIG. 3 may be used.

Next, a resist mask is formed over the conductive film 222 through aphotolithography step, and the conductive film 222 is processed into aback gate electrode 224 (see FIG. 9B).

Through the above steps, a bottom-gate top-contact transistor 252 can bemanufactured.

Next, a protective insulating film 226 covering the back gate electrode224 and the interlayer insulating film 216 may be formed (see FIG. 9C).As the protective insulating film 226, a silicon nitride film, a siliconnitride oxide film, or an aluminum oxide film may be formed by asputtering method or a CVD method. By the provision of the protectiveinsulating film 226, entry of hydrogen from the outside into thetransistor 252 can be suppressed.

According to this embodiment, hydrogen in the film and the interfacebetween the films can be removed, so that a transistor with lessvariation in the threshold voltage and stable electrical characteristicsis provided. Further, hydrogen in a film in contact with an oxidesemiconductor film can be reduced, whereby entry of hydrogen into theoxide semiconductor film can be suppressed. Consequently, asemiconductor device having a transistor with favorable electricalcharacteristics and high reliability can be provided.

Further, the number of apparatuses needed for manufacture of atransistor can be reduced with the use of a multi-chamber film formationapparatus.

(Embodiment 3)

In this embodiment, description is made on a method for manufacturing abottom-gate top-contact transistor using an oxide semiconductor withless entry of hydrogen into a film and an interface between films withreference to FIGS. 10A to 10E. The method is different from that inEmbodiment 2. Note that steps up to and including those in the flow ofFIG. 6 and FIGS. 7A to 7C are similar to those in Embodiment 2.

Manufacturing steps up to and including those in FIGS. 10A and 10B aresimilar to those in the manufacturing steps up to and including those inFIGS. 8A and 8B.

After the oxide semiconductor film 206 is formed, the substrate 200 issubjected to heat treatment or plasma treatment. By subjecting the oxidesemiconductor film 206 to the heat treatment or the plasma treatment,the oxide semiconductor film 206 can be dehydrated or dehydrogenated.The heat treatment or the plasma treatment is performed at a temperaturehigher than or equal to 150° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 250° C. and lower than orequal to 470° C. in an oxidizing atmosphere, an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere.

An oxide conductive film 328 and a conductive film 308 are formed overthe oxide semiconductor film 206 (see FIG. 10C). The oxide conductivefilm 328 may have a structure similar to that of the oxide conductivefilm 128. Note that the oxide conductive film 328 is not necessarilyprovided. Further, the conductive film 308 is processed to form a sourceelectrode 308 a and a drain electrode 308 b later. The conductive film308 may have a structure similar to that of the conductive film servingas the source electrode 108 a and the drain electrode 108 b.

Here, the steps from the formation of the gate insulating film 212 tothe formation of the conductive film 308 are preferably performedwithout exposure to the air. Note that the multi-chamber film formationapparatus shown in FIG. 3 can be used for the steps from the formationof the gate insulating film 212 to the formation of the conductive film308.

Next, a resist mask is formed over the conductive film 308 through aphotolithography step and the conductive film 328 and the oxidesemiconductor film 206 are each processed to have an island shape (seeFIG. 10D).

Next, a resist covering the conductive film 308, the oxide conductivefilm 328, the oxide semiconductor film 206, and the substrate 200 isapplied, a resist mask is formed through a photolithography step, andthe conductive film 308 and the oxide conductive film 328 are processedto form the source electrode 308 a and the drain electrode 308 b. Atthis time, the oxide conductive film 328 is formed between the sourceelectrode 308 a and the oxide semiconductor film 206 and between thedrain electrode 308 b and the oxide semiconductor film 206 (see FIG.10E). Although not shown, part of the oxide semiconductor film 206between the source electrode 308 a and the drain electrode 308 b may beetched.

Through the above steps, a bottom-gate top-contact transistor 350 can bemanufactured.

Following the above steps, an interlayer insulating film, a back gateelectrode, and a protective insulating film may be formed in a mannersimilar to that of the transistor 252. Note that the oxide semiconductorfilm 206 exposed before the formation of an interlayer insulating filmmay be subjected to heat treatment at a temperature higher than or equalto 200° C. and lower than or equal to 500° C. in an oxidizingatmosphere, an inert atmosphere, a reduced-pressure atmosphere, or a dryair atmosphere. Then, the interlayer insulating film is formed withoutexposure to the air.

According to this embodiment, the steps from the formation of the gateinsulating film 212 to the formation of the conductive film 308 can beperformed without exposure to the air, hydrogen in the film and theinterface between the films can be removed, so that the electricalcharacteristics and the reliability of the transistor can be furtherimproved than those in Embodiment 2.

Further, the number of apparatuses needed for manufacture of atransistor can be reduced with the use of a multi-chamber film formationapparatus.

(Embodiment 4)

In this embodiment, a method for forming an insulating film releasingoxygen is described with reference to FIGS. 2A to 2D and FIG. 18.

FIG. 18 shows a structure in which the film formation apparatus in FIG.3 is provided with an ion implantation chamber 17.

In the ion implantation chamber 17, ion doping or ion implantation canbe performed.

Note that, in this embodiment, film formation, ion implantation, andheat treatment or plasma treatment are performed successively in avacuum state as much as possible. The method for forming an insulatingfilm releasing oxygen by using the film formation apparatus in FIG. 18is described below.

First, the substrate 100 is introduced into the load lock chamber 12 a.Next, the substrate 100 is transferred to the substrate processingchamber 15, and hydrogen adsorbed to the substrate 100 is removedthrough first heat treatment, plasma treatment, or the like in thesubstrate processing chamber 15. Here, the first heat treatment isperformed at temperature higher than or equal to 100° C. and lower thanthe strain point of the substrate in an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. Further, for theplasma treatment, rare gas, oxygen, nitrogen, or nitrogen oxide (e.g.,nitrous oxide, nitrogen monoxide, or nitrous oxide) is used. Then, thesubstrate 100 is transferred to the film formation chamber 10 a and thebase insulating film 102 is formed by a sputtering method to have athickness greater than or equal to 50 nm and less than or equal to 500nm, preferably greater than or equal to 200 nm and less than or equal to400 nm (see FIG. 2B). Then, oxygen whose mass number is 16 (¹⁶O), oxygenwhose mass number is 18 (¹⁸O), or ¹⁶O and ¹⁸O are implanted to the baseinsulating film 102 by an ion doping method or an ion implantationmethod. At this time, when an ion doping method is used, hydrogen isalso implanted to the base insulating film 102. For this reason, an ionimplantation method is preferably used. Then, after the substrate 100 istransferred to the substrate processing chamber 15, second heattreatment may be performed at temperature higher than or equal to 150°C. and lower than or equal to 280° C., preferably higher than or equalto 200° C. and lower than or equal to 250° C. in an inert atmosphere, areduced-pressure atmosphere, or a dry air atmosphere. Through the secondheat treatment, hydrogen can be removed from the substrate 100 and thebase insulating film 102. Note that the second heat treatment isperformed at temperature at which hydrogen is removed from the baseinsulating film 102 but as less oxygen as possible is released. Next,the substrate 100 is transferred to the film formation chamber 10 b andthe oxide semiconductor film 106 is formed by a sputtering method (seeFIG. 2C). Then, after the substrate 100 is transferred to the substrateprocessing chamber 15, third heat treatment may be performed attemperature greater than or equal to 250° C. and lower than or equal to470° C. in an inert atmosphere, a reduced-pressure atmosphere, or a dryair atmosphere so that hydrogen is removed from the oxide semiconductorfilm while oxygen is supplied from the base insulating film 102 to theoxide semiconductor film. Note that the third heat treatment isperformed at higher temperature than that of the second heat treatmentby 5° C. or more. By use of the film formation apparatus in FIG. 18 inthis manner, the manufacturing process can proceed with less entry ofhydrogen during film formation.

Through the above, according to Embodiment 1 and the like, asemiconductor device using an oxide semiconductor with less variation inelectrical characteristics can be provided. Further, a semiconductordevice with high reliability can be provided.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

(Embodiment 5)

In this embodiment, a method for manufacturing a transistor which isused for a pixel in a liquid crystal display device and for whichreduced number of photomasks and reduced number of photolithographysteps are used is described with reference to FIG. 11, FIGS. 12A and12B, and FIGS. 13A to 13F.

FIG. 11 shows a circuit configuration of a pixel 442. The pixel 442includes a transistor 450, a liquid crystal element 446, and a capacitor448. A gate electrode of the transistor 450 is electrically connected toa wiring 444 and one of a source electrode and a drain electrode of thetransistor 450 is electrically connected to a wiring 440. The other ofthe source electrode and the drain electrode of the transistor 450 iselectrically connected to one electrode of the liquid crystal element446 and one electrode of the capacitor 448. The other electrode of theliquid crystal element 446 and the other electrode of the capacitor 448are electrically connected to an electrode 445. The potential of theelectrode 445 may be set to a fixed potential such as 0V, GND, or acommon potential.

The transistor 450 has a function of selecting whether an image signalsupplied from the wiring 440 is input to the liquid crystal element 446.When a signal for turning on the transistor 450 is supplied to thewiring 444, an image signal from the wiring 440 is supplied to theliquid crystal element 446 through the transistor 450. The lighttransmittance of the liquid crystal element 446 is controlled dependingon a supplied image signal (potential). The capacitor 448 has a functionof a storage capacitor (also referred to as a Cs capacitor) for storinga potential supplied to the liquid crystal element 446. The capacitor448 is not necessarily provided. However, by provision of the capacitor448, a change in the potential applied to the liquid crystal element 446due to a current (off-state current) flowing between the sourceelectrode and the drain electrode of the transistor 450 which is turnedoff can be suppressed.

An oxide semiconductor is used as a semiconductor where a channel of thetransistor 450 is formed. In a transistor obtained by processing anoxide semiconductor with an energy gap of greater than or equal to 3.0eV under appropriate conditions, the off-state current of the oxidesemiconductor can be less than or equal to 100 zA (1×10⁻¹⁹ A/μm) or lessthan or equal to 10 zA (1×10⁻²⁰ A/μm), preferably less than or equal to1 zA (1×10⁻²¹ A/μm) at operating temperature (e.g., 25° C.). For thisreason, the potential applied to the liquid crystal element 446 can bestored without provision of the capacitor 448. Further, a liquid crystaldisplay device with low power consumption can be provided.

Next, an example of a structure of the pixel 442 shown in FIG. 11 isdescribed with reference to FIGS. 12A and 12B. FIG. 12A is a top viewshowing a plan structure of the pixel 442, and FIG. 12B is across-sectional view showing a stacked structure of the transistor 450.Note that A1-A2 in FIG. 12B corresponds to the cross section along lineA1-A2 in FIG. 12A.

In the transistor 450 described in this embodiment, a drain electrode408 b is partly surrounded by a source electrode 408 a having a U-shape(a C-shape, a reversed C-shape, or a horseshoe shape). With such ashape, even when the area of the transistor is small, a sufficientchannel width can be secured, so that the on-state current of thetransistor can be increased.

When a large amount of parasitic capacitance is generated between thegate electrode 414 and the drain electrode 408 b electrically connectedto the pixel electrode 431, the liquid crystal element 446 is morelikely to be affected by a feedthrough; therefore, a potential suppliedto the liquid crystal element 446 cannot be accurately stored, whichleads to deterioration in display quality. As described in thisembodiment, by employing the shape in which the drain electrode 408 b issurrounded by the source electrode 408 a having a U-shape, a sufficientchannel width can be secured and the parasitic capacitance generatedbetween the drain electrode 408 b and the gate electrode 414 can bereduced, so that display quality of the liquid crystal display devicecan be improved.

In the cross section along A1-A2, a base insulating film 402 is formedover a substrate 400 and the gate electrode 414 is formed over the baseinsulating film 402. A gate insulating film 412 and an oxidesemiconductor film 406 are formed over the gate electrode 414. Thesource electrode 408 a and the drain electrode 408 b are formed over theoxide semiconductor film 406. An interlayer insulating film 416 isformed over the source electrode 408 a and the drain electrode 408 b andin contact with part of the oxide semiconductor film 406. The pixelelectrode 431 is formed over the interlayer insulating film 416. Thepixel electrode 431 is electrically connected to the drain electrode 408b through a contact hole 415 formed in the interlayer insulating film416.

Further, parts of the gate insulating film 412, the oxide semiconductorfilm 406, and the interlayer insulating film 416 are removed and thepixel electrode 431 is formed in contact with side surfaces of the gateinsulating film 412, the oxide semiconductor film 406, and theinterlayer insulating film 416. In this embodiment, an i-type(intrinsic) or substantially i-type oxide semiconductor is used for theoxide semiconductor film 406. The i-type or substantially i-type oxidesemiconductor can be regarded as substantially an insulator, so thatwhen the pixel electrode 431 is in contact with an end portion of theoxide semiconductor film 406, a problem of leakage current or the likedoes not occur.

Next, a method for manufacturing the pixel portion of the liquid crystaldisplay device described with reference to FIGS. 12A and 12B isdescribed with reference to FIGS. 13A to 13F.

First, the base insulating film 402 is formed over the substrate 400.The substrate 400 and the base insulating film 402 may have structuressimilar to those of the substrate 100 and the base insulating film 102respectively.

Next, a conductive film is formed over the base insulating film 402, aresist mask is formed through a photolithography step, and theconductive film is processed to form the gate electrode 414 (see FIG.13A). Although not shown, a capacitor wiring 447 and the wiring 444 areformed at the same time. The gate electrode 414 may have a structuresimilar to that of the gate electrode 114.

A material of the conductive film may be similar to that of the gateelectrode 114 shown in Embodiment 1.

Next, the gate insulating film 412 is formed over the gate electrode414. The gate insulating film 412 may have a structure similar to thatof the gate insulating film 112.

The gate insulating film 412 also functions as a protective layer. Thegate electrode 414 including Cu is covered with an insulating filmincluding silicon nitride, whereby diffusion of Cu from the gateelectrode 414 can be prevented.

Next, the oxide semiconductor film 406 is formed over the gateinsulating film 412. The oxide semiconductor film 406 may have astructure similar to that of the oxide semiconductor film 106.

After the formation of the oxide semiconductor film 406, the substrate400 may be subjected to plasma treatment or heat treatment. Byperforming the plasma treatment or the heat treatment on the substrate400, hydrogen in the oxide semiconductor film 406 can be reduced, whichis preferable. The plasma treatment is preferably performed in anoxidizing atmosphere. The heat treatment is performed in an inertatmosphere, a reduced-pressure atmosphere, or an oxidizing atmosphere.The heat treatment temperature is set at higher than or equal to 100° C.and lower than or equal to 400° C., preferably higher than or equal to200° C. and lower than or equal to 350° C., more preferably higher thanor equal to 250° C. and lower than or equal to 300° C.

Next, a conductive film 408 is formed over the oxide semiconductor film406 (see FIG. 13B). The conductive film 408 may have a structure similarto that of the conductive film serving as the source electrode 108 a andthe drain electrode 108 b.

Here, the steps from the formation of the oxide semiconductor film 406to the formation of the conductive film 408 are performed withoutexposure to the air. Without exposure to the air, hydrogen in the filmand the interface between the films can be removed. Note that the stepsfrom the formation of the oxide semiconductor film 406 to the formationof the conductive film 408 may be performed with the use of themulti-chamber film formation apparatus shown in FIG. 3.

Next, a resist mask is formed over the conductive film 408 through aphotolithography step and the conductive film 408 is processed to formthe source electrode 408 a and the drain electrode 408 b. Although notshown, the wiring 440 is formed at the same time (see FIG. 13C).

Next, the interlayer insulating film 416 is formed over the sourceelectrode 408 a and the drain electrode 408 b (see FIG. 13D). Theinterlayer insulating film 416 may have a structure similar to that ofthe interlayer insulating film 216.

Then, a resist mask is formed over the interlayer insulating film 416through a photolithography step and the interlayer insulating film 416,the oxide semiconductor film 406, and the gate insulating film 412 areprocessed. At this time, only part of the interlayer insulating film 416is removed over the drain electrode 408 b, so that the contact hole 415is formed (see FIG. 13E).

At this time, parts of the interlayer insulating film 416, the oxidesemiconductor film 406, and the gate insulating film 412 in a pixelopening portion (a portion in a pixel where wirings or transistors arenot provided) may be left. Note that by removing the parts of theinterlayer insulating film 416 and the oxide semiconductor film 406 asmuch as possible, the transmittance of the pixel in the case where theliquid crystal display device is used as a transmissive liquid crystaldisplay device is improved. Thus, light from a backlight is efficientlytransmitted through the pixel, whereby power consumption can be reducedand display quality can be improved due to improvement in luminance,which is preferable.

Etching of the interlayer insulating film 416, the oxide semiconductorfilm 406, and the gate insulating film 412 may be performed by eitherdry etching or wet etching, or both. A gas containing chlorine (achlorine-based gas such as chlorine (Cl₂), boron trichloride (BCl₃),silicon tetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) can beemployed as an etching gas used for dry etching.

As a dry etching method, a parallel plate reactive ion etching (RIE)method, an inductively coupled plasma (ICP) etching method, or the likecan be used.

In general, etching of a semiconductor film and formation of a contacthole in an insulating film are separately performed through differentphotolithography steps and etching steps. However, according to themanufacturing steps described in this embodiment, the etching of asemiconductor film and the formation of a contact hole can be performedat the same time through one photolithography step and one etching step.Consequently, not only the number of photo masks but also the number ofphotolithography steps can be reduced. In other words, because of thereduced number of photolithography steps, a liquid crystal displaydevice can be manufactured at low cost with high productivity.

Further, according to the manufacturing steps described in thisembodiment, a photoresist is not formed directly on the oxidesemiconductor film. A channel formation region of the oxidesemiconductor film 406 is protected by the interlayer insulating film416, whereby moisture is not attached to the channel formation region ofthe oxide semiconductor film 406 in a peeling step of the photoresist;therefore, variation in the characteristics of the transistor 450 isreduced and the reliability is improved.

Next, a conductive film is formed over the interlayer insulating film416, a resist mask is formed over the conductive film through aphotolithography step, and the conductive film is processed to form thepixel electrode 431 (see FIG. 13F). The pixel electrode 431 can have astructure similar to that of the back gate electrode 224. Note that aback gate electrode may be formed by processing the conductive film. Byprovision of the back gate electrode, the threshold voltage of thetransistor can be controlled.

The pixel electrode 431 is electrically connected to the drain electrode408 b through the contact hole 415.

According to this embodiment, a pixel of a liquid crystal display devicecan be manufactured with a reduced number of photolithography stepscompared to the number of photolithography steps in a conventionalmanufacturing method. Consequently, a liquid crystal display device canbe manufactured at low cost with high productivity.

According to this embodiment, hydrogen in the film and the interfacebetween the films can be removed, so that a transistor with lessvariation in the threshold voltage and stable electrical characteristicsis provided. Further, hydrogen in a film in contact with an oxidesemiconductor film can be reduced, whereby entry of hydrogen into theoxide semiconductor film can be suppressed. Thus, a semiconductor devicehaving a transistor with favorable electrical characteristics and highreliability can be provided.

Further, the number of apparatuses needed for manufacture of atransistor can be reduced with the use of a multi-chamber film formationapparatus.

This embodiment can be freely combined with other embodiments.

(Embodiment 6)

In this embodiment, an example of a transistor capable of high-speedoperation and having high reliability will be described with referenceto FIG. 17.

FIG. 17 is a cross-sectional view of a bottom-gate bottom-contacttransistor 550. The transistor 550 includes a substrate 500, a baseinsulating film 502 provided over the substrate 500, a gate electrode514 provided over the base insulating film 502, a gate insulating film512 covering the gate electrode 514, an oxide semiconductor film 506provided over the gate electrode 514 with the gate insulating film 512interposed therebetween, a source electrode 508 a and a drain electrode508 b which are connected to the oxide semiconductor film 506 through anoxide conductive film 528, a wiring 518 connected to the sourceelectrode 508 a and the drain electrode 508 b through a contact holeformed in the interlayer insulating film 516, a back gate electrode 524provided over the same surface as the wiring 518 and facing the gateelectrode 514 with the oxide semiconductor film 506 interposedtherebetween, and a protective insulating film 526 covering the backgate electrode 524 and the wiring 518 and being partly in contact withthe interlayer insulating film 516. Note that the oxide conductive film528 and/or the base insulating film 502 are/is not necessarily included.

The substrate 500, the gate electrode 514, the gate insulating film 512,the oxide semiconductor film 506, the oxide conductive film 528, and theinterlayer insulating film 516 may have structures similar to those ofthe substrate 200, the gate electrode 214, the gate insulating film 212,the oxide semiconductor film 206, the oxide conductive film 228, and theinterlayer insulating film 216 respectively.

The source electrode 508 a and the drain electrode 508 b are formed of aTi film, a W film, or the like to have a thickness of greater than orequal to 50 nm and less than or equal to 150 nm.

The back gate electrode 524 and the wiring 518 may have a layeredstructure in which an Al film is sandwiched between Ti films or alayered structure in which an Al film is sandwiched between Mo films.With such a structure, the resistance of the wiring 518 can be loweredand the operation speed of the transistor 550 can be improved.

Moreover, the thicknesses of the source electrode 508 a and the drainelectrode 508 b can be reduced, whereby the coverage with the interlayerinsulating film 516 in a step portion formed due to the source electrode508 a and the drain electrode 508 b can be increased. For this reason,the reliability of the transistor can be improved.

Further, the protective insulating film 526 may include an aluminumoxide film, a silicon nitride film, or a silicon nitride oxide film.With such a structure, entry of hydrogen into the transistor 550 fromthe outside can be suppressed.

According to this embodiment, a transistor capable of high-speedoperation and having high reliability can be manufactured.

(Embodiment 7)

One embodiment of a film formation method for an oxide semiconductorfilm that can be used for a semiconductor film of a transistor inEmbodiments 1 to 6 will be described.

An oxide semiconductor film in this embodiment is a crystalline oxidesemiconductor film.

First, a base insulating film is formed over a substrate.

Next, an oxide semiconductor film having a thickness greater than orequal to 1 nm and less than or equal to 50 nm, preferably greater thanor equal to 5 nm and less than or equal to 30 nm is formed over the baseinsulating film. A sputtering method is used for the formation of theoxide semiconductor film. The substrate temperature during the filmformation is higher than or equal to 100° C. and lower than or equal to500° C., preferably higher than or equal to 200° C. and lower than orequal to 400° C., more preferably higher than or equal to 250° C. andlower than or equal to 300° C.

In this embodiment, the oxide semiconductor film is formed to have athickness of 25 nm in an oxygen atmosphere, an argon atmosphere, or amixed atmosphere of argon and oxygen in the conditions where a targetfor an oxide semiconductor (a target for an In—Ga—Zn—O-based oxidesemiconductor containing In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio])is used, the distance between the substrate and the target is 60 mm, thesubstrate temperature is 300° C., the pressure is 0.4 Pa, and the directcurrent (DC) power source is 0.5 kW.

Next, the atmosphere in the chamber in which the substrate is put is setto a nitrogen atmosphere or dry air, and heat treatment may beperformed. The temperature of the heat treatment is higher than or equalto 200° C. and lower than or equal to 750° C., preferably higher than orequal to 250° C. and lower than or equal to 400° C. Crystallinity of thecrystalline oxide semiconductor film can be improved by the heattreatment.

Here, the steps of the formation of the base insulating film, theformation of the crystalline semiconductor film, and the heat treatmenton the substrate are preferably performed without exposure to the airwith the use of the multi-chamber film formation apparatus shown in FIG.3. Without exposure to the air, hydrogen in the film and the interfacebetween the films can be removed.

Further, by the heat treatment, oxygen in the base insulating film isdiffused into an interface between the base insulating film and thecrystalline oxide semiconductor film or the vicinity of the interface(within ±5 nm from the interface), so that oxygen deficiency in thecrystalline oxide semiconductor film and the interface state between thebase insulating film and the crystalline oxide semiconductor film can bereduced.

Note that the crystalline oxide semiconductor film comprises an oxideincluding a crystal with c-axis alignment (also referred to as C-AxisAligned Crystal (CAAC)), which has neither a single crystal structurenor an amorphous structure. Note that the crystalline oxidesemiconductor film partly includes a crystal grain boundary.

In particular, in the transistor in Embodiment 1 in which the oxidesemiconductor of this embodiment is used as an oxide semiconductor film,an electric field is not applied from one surface to the other surfaceof the oxide semiconductor film and current does not flow in thethickness direction of the oxide semiconductor. For this reason, thetransistor has a structure in which current mainly flows along theinterface of the stack of the oxide semiconductor; therefore, even whenthe transistor is irradiated with light or even when a bias-temperature(BT) stress is applied to the transistor, deterioration of electricalcharacteristics is suppressed or reduced.

When a crystalline oxide semiconductor film is used for a transistor,the transistor can have stable electrical characteristics and highreliability.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

(Embodiment 8)

One embodiment of a film formation method for an oxide semiconductorfilm that can be used for a semiconductor film of a transistor inEmbodiments 1 to 6 will be described using FIGS. 14A to 14C.

The oxide semiconductor film of this embodiment has a layered structureincluding a first crystalline oxide semiconductor film and a secondcrystalline oxide semiconductor film thereover which is thicker than thefirst crystalline oxide semiconductor film.

First, a base insulating film 602 is formed over a substrate 600.

Next, a first oxide semiconductor film having a thickness greater thanor equal to 1 nm and less than or equal to 10 nm is formed over the baseinsulating film 602. A sputtering method is used for the formation ofthe first oxide semiconductor film. The substrate temperature during thefilm formation is higher than or equal to 100° C. and lower than orequal to 500° C., preferably higher than or equal to 200° C. and lowerthan or equal to 400° C., more preferably higher than or equal to 250°C. and lower than or equal to 300° C.

In this embodiment, the first oxide semiconductor film having athickness of 5 nm is formed using a target for an oxide semiconductor (atarget for an In—Ga—Zn—O-based oxide semiconductor containing In₂O₃,Ga₂O₃, and ZnO at 1:1:2 [molar ratio]), with a distance between thesubstrate and the target of 60 mm, a substrate temperature of 300° C., apressure of 0.4 Pa, and a direct current (DC) power source of 0.5 kW inan atmosphere of only oxygen, only argon, or argon and oxygen.

Next, the atmosphere in the chamber in which the substrate is put is setto a nitrogen atmosphere or dry air, and first crystallization heattreatment is performed. The temperature of the first crystallizationheat treatment is higher than or equal to 400° C. and lower than orequal to 750° C. A first crystalline oxide semiconductor film 606 a isformed by the first crystallization heat treatment (see FIG. 14A).

Depending on the temperature of the first crystallization heattreatment, the first crystallization heat treatment causescrystallization from a film surface and crystal growth from the filmsurface toward the inside of the film; thus, c-axis aligned crystal isobtained. By the first crystallization heat treatment, the proportionsof zinc and oxygen in the film surface are increased, and one or morelayers of graphene-type two-dimensional crystal including zinc andoxygen and having a hexagonal upper plane are formed at the outermostsurface; the layers grow in the thickness direction to overlap with eachother. By an increase in the temperature of the crystallization heattreatment, the crystal growth proceeds from the surface to the insideand further from the inside to the bottom.

By the first crystallization heat treatment, oxygen in the baseinsulating film 602 is diffused into an interface between the baseinsulating film 602 and the first crystalline oxide semiconductor film606 a or the vicinity of the interface (within ±5 nm from theinterface), so that oxygen deficiency in the first crystalline oxidesemiconductor film 606 a and the interface state between the baseinsulating film 602 and the first crystalline oxide semiconductor film606 a can be reduced.

Next, a second oxide semiconductor film having a thickness greater than10 nm is formed over the first crystalline oxide semiconductor film 606a. In formation of the second crystalline oxide semiconductor film, asputtering method is used, and a substrate temperature is higher than orequal to 100° C. and lower than or equal to 500° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C., morepreferably higher than or equal to 250° C. and lower than or equal to300° C. With a substrate temperature higher than or equal to 100° C. andlower than or equal to 500° C. in the film formation, precursors can bearranged in the oxide semiconductor film formed over and in contact withthe surface of the first crystalline oxide semiconductor film andso-called orderliness can be obtained.

In this embodiment, the second oxide semiconductor film is formed tohave a thickness of 25 nm in an oxygen atmosphere, an argon atmosphere,or a mixed atmosphere of argon and oxygen in the conditions where atarget for an oxide semiconductor (a target for an In—Ga—Zn—O-basedoxide semiconductor containing In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molarratio]) is used, the distance between the substrate and the target is 60mm, the substrate temperature is 400° C., the pressure is 0.4 Pa, andthe direct current (DC) power source is 0.5 kW.

Then, second crystallization heat treatment is performed. Thetemperature of the second crystallization heat treatment is higher thanor equal to 400° C. and lower than or equal to 750° C. A secondcrystalline oxide semiconductor film 606 b is formed by the secondcrystallization heat treatment (see FIG. 14B). Here, the secondcrystalline heat treatment is preferably performed in a nitrogenatmosphere, an oxygen atmosphere, or a mixed atmosphere of argon andoxygen so that the density of the second crystalline oxide semiconductorfilm can be increased and the number of defects therein can be reduced.By the second crystallization heat treatment, crystal growth proceeds inthe thickness direction with the use of the first crystalline oxidesemiconductor film 606 a as a nucleus, that is, crystal growth proceedsfrom the bottom to the inside; thus, the second crystalline oxidesemiconductor film 606 b is formed.

It is preferable that the steps from the formation of the baseinsulating film 602 to the second crystalline heat treatment beperformed successively without exposure to the air. For example, themulti-chamber film formation apparatus shown in FIG. 3 may be used. Theatmospheres of the film formation chambers 10 a, 10 b, and 10 c, thetransfer chamber 13, and the substrate processing chamber 15 arepreferably controlled so as to hardly contain hydrogen and moisture(i.e., as an inert atmosphere, a reduced-pressure atmosphere, or a dryair atmosphere). For example, a preferable atmosphere is a dry nitrogenatmosphere in which the dew point of moisture is lower than or equal to−40° C., preferably lower than or equal to −50° C. An example of aprocedure of the manufacturing steps with use of the film formationapparatus shown in FIG. 3 is as follows. The substrate 600 is firsttransferred from the substrate supply chamber 11 to the substrateprocessing chamber 15 through the load lock chamber 12 a and thetransfer chamber 13; hydrogen adhering to the substrate 600 is removedby vacuum baking or the like in the substrate processing chamber 15; thesubstrate 600 is then transferred to the film formation chamber 10 cthrough the transfer chamber 13; and the base insulating film 602 isformed in the film formation chamber 10 c. Then, the substrate 600 istransferred to the film formation chamber 10 a through the transferchamber 13 without exposure to the air, and the first oxidesemiconductor film having a thickness of 5 nm is formed in the filmformation chamber 10 a. Then, the substrate 600 is transferred to thesubstrate processing chamber 15 though the transfer chamber 13 withoutexposure to the air and first crystallization heat treatment isperformed. Then, the substrate 600 is transferred to the film formationchamber 10 a through the transfer chamber 13, and the second oxidesemiconductor film having a thickness greater than 10 nm is formed inthe film formation chamber 10 a. Then, the substrate 600 is transferredto the substrate processing chamber 15 through the transfer chamber 13,and second crystallization heat treatment is performed. As describedabove, with use of the film formation apparatus shown in FIG. 3, amanufacturing process can proceed without exposure to the air. Further,after a stack of the base insulating film 602, the first crystallineoxide semiconductor film, and the second crystalline oxide semiconductorfilm is formed, in the film formation chamber 10 b, a conductive filmfor forming a source electrode and a drain electrode can be formed overthe second crystalline oxide semiconductor film with use of a metaltarget, without exposure to the air. Note that the first crystallineoxide semiconductor film and the second crystalline oxide semiconductorfilm may be formed in separate film formation chambers for improvementof the throughput.

Next, a stack of oxide semiconductor films including the firstcrystalline oxide semiconductor film 606 a and the second crystallineoxide semiconductor film 606 b is processed to form an oxidesemiconductor film 606 including the island-shaped stack of the oxidesemiconductor films (see FIG. 14C). In the drawings, the interfacebetween the first crystalline oxide semiconductor film 606 a and thesecond crystalline oxide semiconductor film 606 b is indicated by adashed line for description of the stack of the oxide semiconductorfilms; however, the interface is actually not distinct and is shown foreasy understanding.

The stack of the oxide semiconductor films can be processed by beingetched after a mask having a desired shape is formed over the stack ofthe oxide semiconductor films. The mask may be formed by a method suchas photolithography or an ink jet method.

Further, one feature of the first crystalline oxide semiconductor filmand the second crystalline oxide semiconductor film obtained by theabove formation method is that they have c-axis alignment. However, thefirst crystalline oxide semiconductor film and the second crystallineoxide semiconductor film comprise CAAC. Note that the first crystallineoxide semiconductor film and the second crystalline oxide semiconductorfilm partly include a crystal grain boundary.

Note that the first crystalline oxide semiconductor film and the secondcrystalline oxide semiconductor film are each formed using an oxidematerial including at least Zn. For example, a four-component metaloxide such as an In—Al—Ga—Zn—O-based material or an In—Sn—Ga—Zn—O-basedmaterial; a three-component metal oxide such as an In—Ga—Zn—O-basedmaterial, an In—Al—Zn—O-based material, an In—Sn—Zn—O-based material, aSn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or aSn—Al—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, or a Zn—Mg—O-based material; a Zn—O-based material; or thelike can be used. Further, an In—Si—Ga—Zn—O-based material, anIn—Ga—B—Zn—O-based material, or an In—B—Zn—O-based material may be used.In addition, the above materials may contain SiO₂. Here, for example, anIn—Ga—Zn—O-based material means an oxide material including indium (In),gallium (Ga), and zinc (Zn), and there is no particular limitation onthe composition ratio. Further, the In—Ga—Zn—O-based material maycontain an element other than In, Ga, and Zn.

Without limitation to the two-layer structure in which the secondcrystalline oxide semiconductor film is formed over the firstcrystalline oxide semiconductor film, a stack structure of three or morelayers may be formed by repeatedly performing a process of filmformation and crystallization heat treatment for forming a thirdcrystalline oxide semiconductor film after the second crystalline oxidesemiconductor film is formed.

The oxide semiconductor film 606 formed of the stack of the oxidesemiconductor films formed by the above manufacturing method can be usedas appropriate for a transistor which can be applied to thesemiconductor device disclosed in this specification (e.g., thetransistors 150, 250, 252, 350, and 450).

In the transistor 150 according to Embodiment 1, in which the stack ofthe oxide semiconductor films of this embodiment is used as the oxidesemiconductor film 606, an electric field is not applied from onesurface to the other surface of the oxide semiconductor film and currentdoes not flow in the thickness direction (from one surface to the othersurface; specifically, in the vertical direction in FIG. 4B) of thestack of the oxide semiconductor films. The transistor has a structurein which current mainly flows along the interface of the stack of theoxide semiconductor films; therefore, even when the transistor isirradiated with light or even when a bias-temperature (BT) stress isapplied to the transistor, deterioration of electrical characteristicsis suppressed or reduced.

By using a stack of a first crystalline oxide semiconductor film and asecond crystalline oxide semiconductor film, like the oxidesemiconductor film 606, a transistor having stable electricalcharacteristics and high reliability can be realized.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Further, the number of apparatuses needed for manufacture of atransistor can be reduced with the use of a multi-chamber film formationapparatus.

(Embodiment 9)

One mode of a display device using the transistor exemplified inEmbodiments 1 to 6 is shown in FIGS. 15A and 15B.

FIG. 15A is a top view of a panel. In the panel, a transistor 750 and aliquid crystal element 713, which are formed over a first substrate 701,are sealed between the first substrate 701 and a second substrate 706 bya sealant 705. FIG. 15B corresponds to a cross sectional view along M-Nin FIG. 15A.

The sealant 705 is provided so as to surround a pixel portion 702provided over the first substrate 701. The second substrate 706 isprovided over the pixel portion 702. Thus, the pixel portion 702 issealed together with a liquid crystal layer 708 by the first substrate701, the sealant 705, and the second substrate 706.

Further, an input terminal 720 is provided in a region that is differentfrom a region surrounded by the sealant 705 over the first substrate 701and is connected to flexible printed circuits (FPCs) 718 a and 718 b.The FPC 718 a is electrically connected to a signal line driver circuit703 separately manufactured over a different substrate and the FPC 718 bis electrically connected to a scan line driver circuit 704 separatelymanufactured over a different substrate. Various signals and potentialssupplied to the pixel portion 702 are supplied from the signal linedriver circuit 703 and the scan line driver circuit 704 through the FPCs718 a and 718 b.

Note that a connection method of a driver circuit which is separatelyformed over a different substrate is not particularly limited, and achip on glass (COG) method, a wire bonding method, a tape carrierpackage (TCP) method, a tape automated bonding (TAB) method, or the likecan be used.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) can beused. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

The display device shown in FIG. 15B includes an electrode 715 and awiring 716. The electrode 715 and the wiring 716 are electricallyconnected to a terminal included in a FPC 718 a through an anisotropicconductive film 719.

The electrode 715 is formed using the same conductive film as the firstelectrode 730. The wiring 716 is formed using the same conductive filmas a source electrode and a drain electrode of the transistor 750.

Note that in this embodiment, the transistor 750 has a structure similarto that of the transistor 450 described in Embodiment 4; however, it isneedless to say that the structure of the transistor is not limitedthereto. The transistor may be replaced with the transistor manufacturedin any of Embodiments 1 to 6 as appropriate. The transistor 750 providedin the pixel portion 702 is electrically connected to a display elementto form a display panel. There is no particular limitation on the kindof display element as long as display can be performed, and a variety ofkinds of display elements can be employed.

FIGS. 15A and 15B show an example of a liquid crystal display deviceusing a liquid crystal element as a display element. In FIGS. 15A and15B, the liquid crystal element 713 is a display element including thefirst electrode 730, a second electrode 731, and the liquid crystallayer 708. Note that insulating films 732 and 733 serving as alignmentfilms are provided so that the liquid crystal layer 708 is interposedtherebetween. The second electrode 731 is provided on the secondsubstrate 706 side, and the first electrode 730 and the second electrode731 are stacked with the liquid crystal layer 708 provided therebetween.

Further, a reference numeral 735 is a columnar spacer formed of aninsulating film over the second substrate 706 in order to control thethickness (a cell gap) of the liquid crystal layer 708. Alternatively, aspherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. The liquid crystal composition whichincludes a liquid crystal showing a blue phase and a chiral agent has asmall response time of 1 msec or less, has optical isotropy, which makesthe alignment process unneeded, and has a small viewing angledependence. In addition, since an alignment film does not need to beprovided and rubbing treatment is unnecessary, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device can be reduced in themanufacturing process. Thus, productivity of the liquid crystal displaydevice can be improved.

The specific resistivity of the liquid crystal material is greater thanor equal to 1×10⁹ Ω·cm, preferably greater than or equal to 1×10¹¹ Ω·cm,more preferably greater than or equal to 1×10¹² Ω·cm. The value of thespecific resistivity in this specification is measured at 20° C.

The size of storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charge can be held for apredetermined period. By using a transistor including an oxidesemiconductor for a semiconductor film in which a channel region isformed, it is enough to provide a storage capacitor having a capacitancethat is ⅓ or less, preferably ⅕ or less of a liquid crystal capacitanceof each pixel.

The current in an off state (the off-state current) of the transistorusing an oxide semiconductor film used in this embodiment can be madesmall. Consequently, an electrical signal such as an image signal can beheld for a longer period in the pixel, and a writing interval can be setlonger in an on state. Consequently, frequency of refresh operation canbe reduced, which leads to an effect of suppressing power consumption.Further, the transistor using an oxide semiconductor film can store apotential applied to a liquid crystal element without a storagecapacitor.

The field-effect mobility of the transistor using an oxide semiconductorfilm used in this embodiment can be relatively high, whereby high-speedoperation is possible. Therefore, by using the transistor in a pixelportion of a liquid crystal display device, a high-quality image can beprovided. In addition, since the transistors can be separately providedin a driver circuit portion and a pixel portion over one substrate, thenumber of components of the liquid crystal display device can bereduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. The vertical alignment mode is a method of controllingalignment of liquid crystal molecules of a liquid crystal display panel,in which liquid crystal molecules are aligned vertically to a panelsurface when no voltage is applied. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an advancedsuper-view (ASV) mode, and the like can be used. Moreover, it ispossible to use a method called domain multiplication or multi-domaindesign, in which a pixel is divided into some regions (subpixels) andmolecules are aligned in different directions in their respectiveregions.

In the liquid crystal display device, a black matrix (a light-blockinglayer), an optical element (an optical substrate) such as a polarizingelement, a retardation element, or an anti-reflection element, and thelike are provided as appropriate. For example, circular polarization maybe employed by using a polarizing substrate and a retardation substrate.In addition, a backlight, a side light, or the like may be used as alight source.

In addition, it is possible to employ a time-division display method(also called a field-sequential driving method) with the use of aplurality of light-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue respectively). For example, R, G, B, and W (W corresponds towhite), or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween respective dots of color elements. However, one embodiment ofthe present invention is not limited to a color liquid crystal displaydevice and can be applied to a monochrome liquid crystal display device.

In FIGS. 15A and 15B, as the first substrate 701 and the secondsubstrate 706, flexible substrates, for example, plastic substrateshaving a light-transmitting property or the like can be used, as well asglass substrates. As plastic, a fiberglass-reinforced plastics (FRP)plate, a polyvinyl fluoride (PVF) film, a polyester film, or an acrylicresin film can be used. Further, a sheet with a structure in which analuminum foil is sandwiched between PVF films or polyester films can beused.

The liquid crystal display device performs display by transmitting lightfrom a light source or a display element. Therefore, the substrate andthe thin films such as the insulating film and the conductive filmprovided for the pixel portion where light is transmitted havelight-transmitting properties with respect to light in the visible-lightwavelength range.

The first electrode and the second electrode (each of which may becalled a pixel electrode, a common electrode, a counter electrode, orthe like) for applying voltage to the display element may havelight-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 730 and the second electrode 731 can be formed usinga light-transmitting conductive material such as indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, ITO, indium zinc oxide, or indium tin oxide to which siliconoxide is added. Further, a material formed of 1 to 10 graphene sheetsmay be used.

One of the first electrode 730 and the second electrode 731 can beformed of one or more kinds of materials selected from metals such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); alloys of these metals; and nitrides of these metals.

The first electrode 730 and the second electrode 731 can be formed usinga conductive composition including a conductive macromolecule (alsoreferred to as a conductive polymer). As the conductive macromolecule, aso-called π-electron conjugated conductive can be used. For example,polyaniline or a derivative thereof, polypyrrole or a derivativethereof, polythiophene or a derivative thereof, and a copolymer of twoor more of aniline, pyrrole, and thiophene or a derivative thereof canbe given.

Since a transistor is easily broken by static electricity or the like, aprotection circuit is preferably provided. The protection circuit ispreferably formed using a nonlinear element.

As described above, by using any of the transistors exemplified inEmbodiments 1 to 7, a highly reliable liquid crystal display device canbe provided. Note that the transistors described in Embodiments 1 to 7can be applied to not only semiconductor devices having the displayfunctions described above but also semiconductor devices having avariety of functions, such as a power device which is mounted on a powersupply circuit, a semiconductor integrated circuit such as LSI, and asemiconductor device having an image sensor function of readinginformation of an object.

This embodiment can be freely combined with other embodiments.

(Embodiment 10)

A semiconductor device which is one embodiment of the present inventioncan be applied to a variety of electronic devices (including gamemachines). Examples of electronic devices are a television set (alsoreferred to as a television or a television receiver), a monitor of acomputer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone handset (alsoreferred to as a mobile phone or a mobile phone device), a portable gamemachine, a portable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic devices each including the semiconductor devicedescribed in the above embodiment will be described.

FIG. 16A shows a laptop personal computer including a main body 801, ahousing 802, a display portion 803, a keyboard 804, and the like. Byapplying the semiconductor device described in Embodiments 1 to 8, thelaptop personal computer can have high reliability.

FIG. 16B shows a portable information terminal (PDA) which includes adisplay portion 813, an external interface 815, an operation button 814,and the like in a main body 811. A stylus 812 is included as anaccessory for operation. By applying the semiconductor device describedin Embodiments 1 to 8, the portable information terminal (PDA) can havehigher reliability.

FIG. 16C shows an example of an e-book reader. For example, the e-bookreader 820 includes two housings, a housing 821 and a housing 822. Thehousings 821 and 822 are bound with each other by an axis portion 825,along which the e-book reader 820 can be opened and closed. With such astructure, the e-book reader 820 can operate like a paper book.

A display portion 823 and a display portion 824 are incorporated in thehousing 821 and the housing 822 respectively. The display portion 823and the display portion 824 may display one image or different images.In the structure where the display portions display different imagesfrom each other, for example, the right display portion (the displayportion 823 in FIG. 16C) can display text and the left display portion(the display portion 824 in FIG. 16C) can display images. By applyingthe semiconductor device described in Embodiments 1 to 8 can be, thee-book reader 820 can have high reliability.

Further, in FIG. 16C, the housing 821 is provided with an operationportion and the like. For example, the housing 821 is provided with apower switch 826, operation keys 827, a speaker 828, and the like. Withthe operation keys 827, pages can be turned. Note that a keyboard, apointing device, or the like may also be provided on the surface of thehousing, on which the display portion is provided. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, orthe like), a recording medium insertion portion, and the like may beprovided on the back surface or the side surface of the housing.Further, the e-book reader 820 may have a function of an electronicdictionary.

The e-book reader 820 may send and receive information wirelessly.Through wireless communication, desired book data or the like can bepurchased and downloaded from an electronic book server.

FIG. 16D shows a mobile phone, which includes two housings, a housing830 and a housing 831. The housing 831 includes a display panel 832, aspeaker 833, a microphone 834, a pointing device 836, a camera lens 837,an external connection terminal 838, and the like. The housing 830includes a solar cell 840 for charging of the portable phone, anexternal memory slot 841, and the like. In addition, an antenna isincorporated in the housing 831. By applying the semiconductor devicedescribed in Embodiments 1 to 8, the mobile phone can have highreliability.

Further, the display panel 832 is provided with a touch panel. Aplurality of operation keys 835 that are displayed as images are shownby dashed lines in FIG. 16D. A boosting circuit by which a voltageoutput from the solar cell 840 is increased to be sufficiently high foreach circuit is also provided.

The display panel 832 changes the orientation of display as appropriatedepending on the application mode. Further, the camera lens 837 isprovided on the same surface as the display panel 832, and thus it canbe used as a video phone. The speaker 833 and the microphone 834 can beused for operations such as video calls, sound recording, and playbackwithout being limited to the voice call function. Further, the housing830 and the housing 831 in a state where they are developed as shown inFIG. 16D can shift by sliding so that one is lapped over the other;therefore, the size of the mobile phone can be reduced, which makes themobile phone suitable for being carried.

The external connection terminal 838 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Furthermore, alarge amount of data can be stored and transferred by inserting arecording medium into the external memory slot 841.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 16E shows a digital video camera, which includes a main body 851, adisplay portion (A) 857, an eyepiece 853, an operation switch 854, adisplay portion (B) 855, a battery 856, and the like. By applying thesemiconductor device described in Embodiments 1 to 8, the digital videocamera can have high reliability.

FIG. 16F shows an example of a television set. In the television set860, a display portion 863 is incorporated in a housing 861. The displayportion 863 can display images. Here, the housing 861 is supported by astand 865. By applying the semiconductor device described in Embodiments1 to 8, the television set 860 can have high reliability.

The television set 860 can be operated by an operation switch of thehousing 861 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 860 is provided with a receiver, a modem,and the like. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed.

The structures, the methods, and the like described in this embodimentmay be combined as appropriate with any of the structures, the methods,and the like described in the other embodiments.

EXPLANATION OF REFERENCE

10 a: film formation chamber, 10 b: film formation chamber, 10 c: filmformation chamber, 11: substrate supply chamber, 12 a: load lockchamber, 12 b: load lock chamber, 13: transfer chamber, 14: cassetteport, 15: substrate processing chamber, 100: substrate, 102: baseinsulating film, 106: oxide semiconductor film, 108 a: source electrode,108 b: drain electrode, 112: gate insulating film, 114: gate electrode,128: oxide conductive film, 150: transistor, 200: substrate, 202: baseinsulating film, 206: oxide semiconductor film, 208 a: source electrode,208 b: drain electrode, 212: gate insulating film, 213: conductive film,214: gate electrode, 216: interlayer insulating film, 222: conductivefilm, 224: back gate electrode, 226: protective insulating film, 228:oxide conductive film, 250: transistor, 252: transistor, 308: conductivefilm, 308 a: source electrode, 308 b: drain electrode, 328: oxideconductive film, 350: transistor, 400: substrate, 402: base insulatingfilm, 406: oxide semiconductor film, 408: conductive film, 408 a: sourceelectrode, 408 b: drain electrode, 412: gate insulating film, 414: gateelectrode, 415: contact hole, 416: interlayer insulating film, 431:pixel electrode, 440: wiring, 442: pixel, 444: wiring, 445: electrode,446: liquid crystal element, 447: capacitor wiring, 448: capacitor, 450:transistor, 500: substrate, 502: base insulating film, 506: oxidesemiconductor film, 508 a: source electrode, 508 b: drain electrode,512: gate insulating film, 514: gate electrode, 516: interlayerinsulating film, 518: wiring, 524: back gate electrode, 526: protectiveinsulating film, 528: oxide conductive film, 550: transistor, 600:substrate, 602: base insulating film, 606: oxide semiconductor film, 606a: crystalline oxide semiconductor film, 606 b: crystalline oxidesemiconductor film, 701: substrate, 702: pixel portion, 703: signal linedriver circuit, 704: scan line driver circuit, 705: sealant, 706:substrate, 708: liquid crystal layer, 713: liquid crystal element, 715:electrode, 716: wiring, 718 a: FPC, 718 b: FPC, 719: anisotropicconductive film, 720: input terminal, 730: electrode, 731: electrode,732: insulating film, 733: insulating film, 750: transistor, 801: mainbody, 802: housing, 803: display portion, 804: keyboard, 811: main body,812: stylus, 813: display portion, 814: operation button, 815: externalinterface, 820: e-book reader, 821: housing, 822: housing, 823: displayportion, 824: display portion, 825: axis portion, 826: power switch,827: operation key, 828: speaker, 830: housing, 831: housing, 832:display panel, 833: speaker, 834: microphone, 835: operation key, 836:pointing device, 837: camera lens, 838: external connection terminal,840: solar cell, 841: external memory slot, 851: main body, 853:eyepiece, 854: operation switch, 855: display portion (B), 856: battery,857: display portion (A), 860: television set, 861: housing, 863:display portion, and 865: stand.

This application is based on Japanese Patent Application serial No.2010-197749 filed with Japan Patent Office on Sep. 3, 2010 and JapanesePatent Application serial No. 2010-287403 filed with Japan Patent Officeon Dec. 24, 2010, the entire contents of which are hereby incorporatedby reference.

The invention claimed is:
 1. A method for manufacturing a semiconductor device, comprising the steps of: forming an insulating film over a substrate; performing dehydration treatment or dehydrogenation treatment on the substrate in a substrate processing chamber; and introducing the substrate into a first film formation chamber without exposure to air, and forming an oxide semiconductor film over the insulating film in the first film formation chamber.
 2. The method according to claim 1, wherein the insulating film is formed by a sputtering method.
 3. The method according to claim 1, further comprising the step of: implanting oxygen into the insulating film by an ion implantation method; wherein the oxygen includes any one of oxygen whose mass number is 16 and oxygen whose mass number is
 18. 4. The method according to claim 1, wherein the oxide semiconductor film is formed by a sputtering method.
 5. The method according to claim 1, wherein the oxide semiconductor film comprises indium, gallium and zinc.
 6. The method for manufacturing a semiconductor device, according to claim 1, further comprising the step of introducing the substrate into a transfer chamber evacuated to be in a vacuum state before performing the dehydration treatment or the dehydrogenation treatment in the substrate processing chamber.
 7. A method for manufacturing a semiconductor device, comprising the steps of: forming a first insulating film over a substrate in a first film formation chamber; performing dehydration treatment or dehydrogenation treatment on the substrate in a substrate processing chamber; introducing the substrate into a second film formation chamber without exposure to air, and forming an oxide semiconductor film over the first insulating film in the second film formation chamber; and introducing the substrate into a third film formation chamber without exposure to air and forming an oxide conductive film over the oxide semiconductor film in the third film formation chamber.
 8. The method for manufacturing a semiconductor device, according to claim 7, further comprising the step of: implanting oxygen into the first insulating film by an ion implantation method before forming the oxide semiconductor film.
 9. The method for manufacturing a semiconductor device, according to claim 7, wherein the dehydration treatment or the dehydrogenation treatment is one of or both of heat treatment and plasma treatment.
 10. The method for manufacturing a semiconductor device, according to claim 9, wherein the dehydration treatment or the dehydrogenation treatment is performed in any one of an inert atmosphere, a reduced-pressure atmosphere, and a dry air atmosphere.
 11. The method for manufacturing a semiconductor device, according to claim 7, wherein the first insulating film is a silicon oxide film including oxygen atoms which are more than twice as many as silicon atoms per unit volume.
 12. The method for manufacturing a semiconductor device, according to claim 7, further comprising the steps of: processing the oxide conductive film and the oxide semiconductor film to form an island-shaped oxide conductive film and an island-shaped oxide semiconductor film; forming a first conductive film over the island-shaped oxide conductive film; and processing the first conductive film and the island-shaped oxide conductive film to form a source electrode and a drain electrode and to form oxide conductive films between the source electrode and the island-shaped oxide semiconductor film and between the drain electrode and the island-shaped oxide semiconductor film.
 13. The method for manufacturing a semiconductor device, according to claim 7, wherein any one of the first film formation chamber to the third film formation chamber is the same as one or more of the other film formation chambers.
 14. The method for manufacturing a semiconductor device, according to claim 7, wherein a leakage rate of the first film formation chamber to the third film formation chamber is less than or equal to 1×10⁻¹⁰ Pa·m³/sec.
 15. The method for manufacturing a semiconductor device, according to claim 7, further comprising the step of introducing the substrate into a transfer chamber evacuated to be in a vacuum state before performing the dehydration treatment or the dehydrogenation treatment in the substrate processing chamber.
 16. A method for manufacturing a semiconductor device, comprising the steps of: forming a first insulating film over a substrate in a first film formation chamber; introducing the substrate into a second film formation chamber and forming a first conductive film over the first insulating film in the second film formation chamber; introducing the substrate into a first substrate processing chamber and performing dehydration treatment or dehydrogenation treatment on the substrate in the first substrate processing chamber after forming the first conductive film; and introducing the substrate into a third film formation chamber without exposure to air, and forming a oxide semiconductor film over the first conductive film in the third film formation chamber.
 17. The method for manufacturing a semiconductor device, according to claim 16, further comprising the step of: implanting oxygen into the first insulating film by an ion implantation method before forming the oxide semiconductor film.
 18. The method for manufacturing a semiconductor device, according to claim 16, wherein the dehydration treatment or the dehydrogenation treatment is one of or both of heat treatment and plasma treatment.
 19. The method for manufacturing a semiconductor device, according to claim 18, wherein the dehydration treatment or the dehydrogenation treatment is performed in any one of an inert atmosphere, a reduced-pressure atmosphere, and a dry air atmosphere.
 20. The method for manufacturing a semiconductor device, according to claim 16, wherein the first insulating film is a silicon oxide film including oxygen atoms which are more than twice as many as silicon atoms per unit volume.
 21. The method for manufacturing a semiconductor device, according to claim 16, further comprising the steps of: introducing the substrate into a second substrate processing chamber, without exposure to air, and performing heat treatment on the substrate at a temperature higher than or equal to 300 ° C. and lower than or equal to 650 ° C. in any one of an inert atmosphere, a reduced-pressure atmosphere, and a dry air atmosphere; introducing the substrate into a fourth film formation chamber without exposure to air, and forming an oxide conductive film over the oxide semiconductor film in the fourth film formation chamber; processing the oxide conductive film and the oxide semiconductor film to form an island-shaped oxide conductive film and an island-shaped oxide semiconductor film; forming a conductive film over the island-shaped oxide conductive film; and processing the conductive film and the island-shaped oxide conductive film to form a source electrode and a drain electrode and to form oxide conductive films between the source electrode and the island-shaped oxide semiconductor film and between the drain electrode and the island-shaped oxide semiconductor film.
 22. The method for manufacturing a semiconductor device, according to claim 21, wherein any one of the first film formation chamber to the fourth film formation chamber is the same as one or more of the other film formation chambers.
 23. The method for manufacturing a semiconductor device, according to claim 21, wherein a leakage rate of the first film formation chamber to the fourth film formation chamber is less than or equal to 1×10⁻¹⁰Pa·m³/sec.
 24. The method for manufacturing a semiconductor device, according to claim 16, further comprising the step of introducing the substrate into a transfer chamber evacuated to be in a vacuum state before performing the dehydration treatment or the dehydrogenation treatment in the first substrate processing chamber. 